Promotions, bonuses and other ideas of a gambling sites is one of their way to attract gamblers to play in their site.
Usually, there is a "must do" or a requirements too in return to get their promotions. Example, minimum required deposit with cashback, etc. 

But for me, the most significant thing I am looking for a partucular gambling site is the fairness. Some said they are provably fair but obviously not if you try to play on their site. Also, the fast withdrawals and deposits matters too.