useless to society...

fashion allows people to optimize their appearance so as to get more positive attention from others. Or, put more crudely, it helps people look good so they can get laid.

Not sure that it's net positive for society as a whole, but individual humans certainly benefit from the fashion industry. Ask anyone who has ever received a compliment on their outfit.

This is true for rich people as well as not so rich people - having spent some time working as a salesman at H&M, I can tell you that lower income members of society (like, for example, H&M employees making minimum wage) are very happy to spend a fair percentage of their income on clothing.