The thing is, you're absolutely right. "Finance" does serve a purpose. I don't think there'd be a stock market where rich people gamble on pieces of companies like poker chips, but the finance industry does serve a purpose under capitalism and a similar thing would be needed: the allocation of huge amounts of society's resources. Financial companies are just capitalist central planners, and they optimize to keep themselves rich and keep capitalism running. Such a thing would be needed under socialism too, but with different incentive structures and goals.