What's the importance that Marx gives to the Class Struggle for the devolopment of societies?

Marx is complex, richly detailed, and comprehensive, but I'll try to give a short, simple, partial response.

Marx was greatly influenced by Hegel and shares his view of historical development. First, human history progresses. It is not cyclical, random, regressive, or linear. It progresses consciously in self-understanding and materially. Major changes come about not because of random external events, like a meteor whacking the dinosaurs, but because of internal contradictions in the structure of the present society. These shifts may happen visibly in "revolutions," or what we might call, in biological evolution, "punctuated equilibria."

In other words, the present order gives way due to some internal contradiction or opposition of social forces. Unlike Hegel, Marx saw these forces as material, basically the way in which a society materially reproduces itself, its bodies, its culture, its knowledge, its tools, its food, its entire material being. The power supply for all this is, simply put, socialized, interdependent human labor, whether physical or mental. This is the great, creative human capacity to develop its freedom from nature, including human nature.

This requires specializations and hierarchies, etc. Yet in all human societies there are also classes. Many, but two above all. The vast majority who "work" and create and produce and those who "own" the means of production, such as land in feudal society. How and why do they own it? Good question! It is not based on how the society actually reproduces itself, but on the traditions, ideologies, religions, laws, modes of thought, institutions etc., that Marxists call the "superstructure."

The productive classes and the dominant "owning" classes have utterly contradictory, conflicting interests. The more that labor gets the less the "owners" get and vice versa. This is shockingly apparent today is the total disconnect between the stock market's major "shareholders" and unemployment and our current pandemic, for example.

Now, it is important to realize "class" does not mean a certain set of people. Feudal lords may become rich merchants after the English, French, an American "Bourgeoise Revolutions," workers today may become small shareholders, etc. The contradiction is between those who can, if they wish, live solely by owning and those who cannot. Their "class" interests are fundamentally opposed.

Now, to answer your question. Why would this drive the direction of social evolution? Simply put, the minority who "own" have a vested interest in the status quo. As a class, they obviously want those laws, ideas, cultural force, governments, etc. that conserve their present ownership to remain unchanged in any truly fundamental way.

Meanwhile, things change. The productive classes grow more globally populous and, at the margins or even the center, more relatively dispossessed. And they produce new technologies that change human powers, productive forces, and the social equations. The static institutions fortifying the "ownership" class appear increasingly irrational, unstable, and expendable.

Thus the inherited land rights of nobles and clergy made less and less sense as shipping, banking, factories, and other bourgeois forces increased. Today, the perfect example is copyright and patent "ownership" of information, music, bandwidths, airwaves, medicines, etc., that are, in terms of our productive forces, nearly free to reproduce. When companies spend more on lawyers than research, a "class contradiction" is revealed.

Marx understood the many complexities, reversals, and in-betweens of social development. And his analysis centered on his own times in the early steam age. But, like Hegel, he was basically an optimist in believing that "reason" would overcome fundamental contradictions, if sometimes violently. We will find out, I guess.

Marx gives a double importance to class struggle in the Manifesto: economic and political.

Economic

The history of all hitherto existing societies is the history of class struggles.

Freeman and slave, patrician and plebeian, lord and serf, guild-master and journeyman, in a word, oppressor and oppressed, stood in constant opposition to one another, carried on in an uninterrupted, now hidden, now open fight, a fight that each time ended, either in a revolutionary reconstitution of society at large, or in the common ruin of the contending classes. (K.Marx & F. Engels, The Communist Manifesto, ed. A.J.P. Taylor, Harmondworth: Penguin, 1967: 79.)

The claim is not made that the history of all hitherto existing societies is only the history of class struggles or that there are only ever two classes. The basic idea is that class struggles are the main motor of history and the principal determinants of social change. All the classes Marx lists are the owners of the forces of production (freeman, patrician, lord, guild-master) or are exploited by the owners of these forces (slave, plebeian, serf, journeyman). In Marx's own day the relevant struggle was, as the first section title makes clear, between bourgeois and proletarian.

Political

The state is used by the dominant class - the owners of the forces of production - to maintain the social order in which oppression and exploitation can take place and safely continue:

The executive of the modern State is but a committee for managing the common affairs of the whole bourgeoisie (Taylor: 82). 'The common affairs' are those of oppression and exploitation.

Politics will, however, put an end to this state of affairs as the oppressed class becomes collectively conscious of its condition and of its ability, through proletarian revolution, to overthrow its bourgeois, capitalist oppressors. What will follow is the dictatorship of the proletariat and the socialist transformation of society - and eventually communism in which the state will disappear since there will be no class-versus-class rule for the state to maintain. There will be only one class, the now liberated proletariat or universal class.

Comment 1 - Marx on 'struggle'

As Marx implies in his phrase, 'now hidden, now open', struggle can take a diversity of forms: strikes, work-to-rules, thefts, protests and demonstrations, and finally revolution.

Comment 2 - Marx on 'class'

If class struggle is to fulfil the historical role that Marx assigns to it, he needs to tell us clearly what a class is. He does this in the case of the proletariat from two perspectives - (1) the structural position of individuals in relation to the forces of production and (2) self-recognition as an oppressed group and accompanying partisanship.

Structural position

...the proletariat is that class of laborers who, having been forcibly separated from the means of production, are condemned to exchange their now fungible capacity to labor - manual as well as mental - for a wage. As a general heuristic framework, the notion of class refers neither to status nor income nor even to the division of labor, but rather to the structural position individuals assume in specific production relations. Quite simply ... a proletarian is one who sells his/her capacity to produce in order to survive, while a bourgeois is one who appropriates this capacity as a commodity in order to valorize capital. (Mohandisi: 73.)

Self-recognition as an oppressed group - and partisanship

In this case, a class no longer is, it only becomes, it is determined not by a position but by a process. The mere fact that a certain group of individuals are made to perform a common role is no longer a sufficient condition for their constitution as a class; class only happens when an agglomeration of separated individuals, all of whom may or may not assume the same general position within a given relation of production, comes together to combat another class. Membership in a class, according to this conception, is determined by active partisanship, which often implies the adoption of a specific perspective, in a given struggle. Class, then, signifies here the formation of a politcal subject. ((Mohandisi: 74.)

Though this second perspective receives more attention in The German Ideology than in the Manifesto, it is clearly implicit in Marx's Manifesto account of the collective consciousness which the proletariat gradually acquires.

Comment 3 - How plausible is Marx's theory of history?

A large topic but a test case has been taken to be that of the French Revolution, which is hard to analyse plausibly in terms of a self-conscious capitalist bourgeoisie rising against the feudal classes. If interested, read Bertel Nygaard (listed below) who handles the issue with finesse, neither wholly for nor wholly against Marx.

References

K. Marx & F. Engels, The Communist Party [Manifesto of the Communist Party], ed. A.J.P. Taylor, Harmondsworth: Penguin, 1967.

Online: http://www.gutenberg.org/ebooks/61

S. Mohandesi, 'Class Consciousness or Class Composition?', Science & Society, January 2013, Vol. 77, No. 1, pp. 72-97: 73-4.

Bertel Nygaard, 'The Meanings of "Bourgeois Revolution": Conceptualizing the French Revolution', Science & Society , Apr., 2007, Vol. 71, No. 2 (Apr., 2007), pp. 146-172.