I notice that some coins that are already in the market are doing bounty campaign again like Credits, Veil Eterbase and among others, do you think it's important for a coins/tokens that are already listed to run a bounty campaign from time to time even if they are already listed in exchanges.
Yes, why not? If you manage bounty campaign you get more users, more attention to your cryptocurrency. Why it is good? More users means higher value of the cryptocurrency.
Good example is: Is Ethereum more used than HB token? Yes, it is, so that is why Ethereum has bigger cappitalization.