The promotions they do will also certainly be able to attract more people who will play on their site. It is hoped that with the right promotion, the profits of a site can increase.
Not only does it attract more people but it also encourages their existing players to continue playing. Even if it's only a classic wagering competition there's still a significant increase in wagering activity.
It depends on the RTP of the platform but if you have a number of spins bonuses when deposit I guess there's a higher chance to get a profit like with the bonuses when you hit a good jackpot this is just my personal preference because I do not totally fan with the slot games and base on my experience RTP is just a small percentage but imagine having 30+ bonus spin that might get profitable if you have a good spins.
I agree those 20-30 free spin rewards you can earn from certain promotions are nice even though they're a hit or miss at times since you could get unlucky and get a bunch of dead spins but at the same time, you could get lucky and hit something decent. But on average i've been getting half of the total value of the free spins so it still provides a nice bump to my bankroll.