Bounty can help both new ico and coins that are already existing in the market, for new ico, it will create awareness for people to invest in the project and for the existing coins on the exchanges, it will make people to know about the coins and their project, it's not really about the market price for the existing coins, it's about letting more people to know about their project.
You perfectly noticed about the existing coins. I think that the bounty companies should be aimed at attracting the audience for existing projects, but not for those that have not been created yet