Why are there multiple kinds of dashes in the English language?

If you look at printing manuals from the mid-19th century (you can find these on Google books, for example Typographia: Or, The Printer's Instructor, from 1857) there wasn't any distinction between em-dashes and en-dashes when used as punctuation.

This book recommends using em-dashes—with no spaces to either side—to set off parenthetical comments, and also using em-dashes for ranges of numbers, like 33—47.

Printers also had dashes available in twice, three times, four times, and maybe six times the length of the em-dash (called a two-em dash and so forth), as well as the en-dash which was half the length of the em-dash. These were intended to be strung together so as to create horizontal rules of arbitrary lengths.

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At some point, I assume that some printer decided that en-dashes—which were readily available because of their use for horizontal rules—looked better than em-dashes for ranges of numbers, e.g. 33–47. Other printers copied him because it did indeed look slightly better. This became enshrined as a tradition, and now standard English punctuation has two lengths of dashes, when one length would serve nearly as well, as it did in the 19th century, and be less confusing.