Bonuses are usually done because gambling sites wanted to attract more gamblers with this. Sites didnt made this just to give away money but they also have it seen in the future. You cannot gambling in just depending on the bonuses you collected because sites knows it can happen and they can possibly have a system which has greater odds if you put your bonus money.
but if you can use those bonuses in a proper way they could bring you a good profit without investing a big sum of money.