No need to necrobump old posts though.
For me, bonuses are for baiting because you will be eager to get more with that balance where in fact you can't clearly cashout and you can only wager. But in the other hand, bonuses is such a big help for raising your bet value and can possibly give you great output. Having bonus is only a headstart for you to have a good result.
As always, they are but it's almost always the way these casinos are expanding. This kind of strategy will likely be done even if they are already large in operation since it's the way to recruit people too. It's one of the ways they can hook users to play or even stay in the casino.