Usually they promise you something like extra money on your first deposit, daily free spins, or something else to get you hooked.
A former coworker claimed to have a scheme on one casino to use those perks for guaranteed free money (of variable amounts per month), buuuut never told me much about it. Given that, it might even have been legit.
Haven’t really tried it myself, I have ADHD so I’m afraid of addictive things. I already have alcohol, nicotine and caffeine in my life, I don’t want to add gambling.