What are the extra human tubes used for?

For the general populace that serve as an energy source for the matrix, life support is required in order to keep them functioning in an optimum state.

While it is evident the brain only requires a single connection to the matrix to be able to enter it (as seen by the single connection into the back of the neck of Zion residents on their ships), in order for the matrix to use the populace as an energy source, the populace must be kept in good health so nutrients, medications (if required) and blood cleansing of intoxicants, carcinogens and other harmful chemicals would be required. It is likely chemicals would be required in order to keep the populace in continual stasis as well.

Energy can also be collected via these tubes either in electrical form or via chemical form for use to power the matrix and dependent systems.

It is worth noting that when Neo was reconnected with the help of the Deus ex Machina in the third movie, he was potentially supplied with more than the single connection used by Zion to access the Matrix. In this scenario, the additional connections could potentially also supply performance enhancing chemicals or other improvements for his connection to the Matrix to give him every edge when combatting Smith for the future of the system. It is likely the System at this time would give every advantage available to get a favourable result.