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The reason for the listed projects promote their services with the bounty campaigns has an obvious target. The team wants to spread the word about their projects to the crypto world and the competition among the projects opens the gap every week. In fact, the target of having a token sale plans serves to collect more funds from the investors. Bounty campaign rewards are not worth to check the rates of the token on coinmarketcap and there are not good sources for selling the project on known exchange.
You are right, of course the project has a very strong reason why to hold a bounty campaign even though it already exists in the market. Of course, to attract more investors to trade with them. This is the main attraction for bounty hunters, when tokens are ready in the market it is a guarantee that their hard work will be valuable, and for developers also profitable because information about their projects and trade is increasingly widespread.