The whales usually don't try these promos as they have enough funds to wager normally on the roobet slots and other games and enjoy their way this way but you see majority is small gambler and they want such competition and promotion as extra reward that binds them to the casino.
But from casino perspective yes they are into gambling business and marketing is the most important part for that as you need to rewards the players and keep them engaging through these ways and keep up your profits.
Exactly, you always need to keep the people motivated to come back to your site so promotions play a vital role. And have a variety of promotions so you keep everybody happy (casino and sports players). Besides that, costumer service also plays a huge role. If you don't care about the players and have a bad support, you will start loosing players when there are issues.