Cashback bonuses are a way to get people to play at the only casino more. There are two main types of cashback bonuses. The signup type and the monthly rewards ones. The signup ones are more generous and have fewer terms attached to them.
Cashback bonuses are in some respects better than the no deposit bonuses, especially if the cashback is 100%. Its essentially risk free play at the casino. With cashback, there are generally no wagering requirements. If there are wagering requirements, then they are pretty small. These are better than deposit bonuses as if you win, you can cash out straight away and if you lose, you get your money back. With a deposit bonus, you have to wager so much before you can cash out, and sometimes that's a lot.
Fewer and few casinos offer cashback nowadays so if you find one, take advantage before they change their minds.