Casino rewards are part of the advertisement method and prices are differ. And some deposit before given the rewards or deposit before you can use it. While some once you registered they give you to test the games in the casino website. And the bigger the reward is the more attractive the site also to gamblers. And all the casinos that I have seen the bonuses can't just be withdraw without wagering game. And as you said the features are almost the same.
One funniest thing casinos do, especially the online casinos they usually send some rewards to their good customers (those that gamble with their platform steady) a decent amount to gamble with. Both some first time gamblers, but those first time gamblers that are just starting to use their platform uses the casino promo codes to get those rewards. Most casinos bonuses are very hard to withdraw because you might have to rollover some specific, so if you are lucky you might be able to withdraw and if you are not lucky you lose everything back to the casino.