Bonuses attract new users to the casino, if they were not useful and useful, then no one would use bonuses, and almost all casinos use many and diverse bonuses and gifts to their users.
Players love the idea that they are getting free money due to the way casinos present their bonuses:
Deposit $100 and get $50 for free.
Deposit each Wednesday and enjoy 100 free spins and so on. The wagering requirements and other terms guarantee that most of the bonus money the casinos give are lost and thus returned by the players. So the casinos have very little to lose but they get new players who deposited something to take advantage of a bonus. Some of them will stick around to continue playing at the site.