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.
Promotions are something that every gambling site must do to get more members to come. But for new members, they will give big promotions because they want to give some special for the new members. And yes, for the old members, they need to pass their requirements to get the promotions. Some of the gambling websites often send me invitations to their site with some promotions but with terms and conditions.
I am sure that every gambling site which has huge members will be fair, but as you said, we don't know if that is real or not. But personally, I never think about that because I don't chase for the winning, and I feel that playing gambling is just for killing my free time.