As you have come to expect the game of roulette when played online will offer you the ability to earn comps on each real money wager you make, and this can return for the bigger player especially quite a tidy sum over time.
One word of caution though once you join an online casino some sites make you join separately their comp or loyalty program, so make sure that you are a member before you start to play for real money or you will be missing out on this free money.
Bonuses should be treated carefully as some online casinos will allow you to play roulette with a new player or money match bonus and some outlaw it.
Check the terms and conditions in great detail as if you use a bonus on roulette and the casino classifies roulette as a disallowed game you will forfeit your winnings and the casino will void all action.
Just to confuse you even more in regard to bonuses, all casinos will make you play through your bonus and/or deposit amount a certain number of times before you can make a cash out and they will increase the number of times you have to play through this amount if you are playing roulette, so check and read the terms and conditions thoroughly until you are 100% sure you can play this game.