The goal of a bounty is to create more awareness and as such attract investors or users. In the case of ICO bounties, they are meant to collect funds for the start of the project. Subsequent investments to keep the project afloat. Veil never did an ICO for example, so funding the project was primarily through tokens sales and appreciation on exchange. Others basically need to keep the trade volume up so as to give value to the tokens. No investment survives without new investors or users.