Because T-shirts are somewhat irregularly shaped when folded. If you stack them with the same orientation, one side is higher than the other. You can alternate the way you lay them but that’s also easier with rolls.

Rolls allow you to take advantage of the efficiency of the honeycomb shape, as well.

Depends on whether you’re folding them in neat squares or hanging them from hangers. The rolls are useful for putting them in luggage, they can be compressed better that way