The players need incentives to keep gambling so the casinos have these bonus options for them and keep them attracted to their platform but they also have strings attached like some wagering requirements so they also gamble with money instead of directly withdrawing that bonus.But it's free money for playing as well so it's all good and each casino has their own terms.
It is mutual relationship between a casino and their users. Benefiting each other and survive, grow together with time. A casino can not use all their capital for promotion because they must keep their platform runs which need to have budget for product developments, maintenance, security, customer service ...
There are easy and difficult promotion events and you can follow
Rollbit Twitter to get notifications on time and have enough time to choose what events match you.