There is no such thing as a free lunch.
What I mean to say, is that people rarely do things without some sort of business reason behind it. The reason may not be an obvious one: it may just be that a developer gets his name in the press and hopes that way to get other well-paying jobs.
So if a DAO is asking for users' money, the users typically have some way to profit from the investment. But that profit model can really vary a lot. It may be direct fees to the investors from interactions (similarly to how liquidity providers get fees in Uniswap) or then something else.
So it really depends on the DAO. But you can be quite sure that users won't give their money out to DAOs for charity.