Online casinos without bonuses will unlikely attract players. It's always nice to receive something on top of what you've spent. Best casino bonuses let players feel that deposits give them extra bets, opportunity to participate in promos and tournaments.
All bonuses (there are lots of them) can be divided into deposit and no deposit categories.
In case of deposit bonuses the condition for receiving them is to transfer money to your account. In case of no-deposit bonus, credits will be awarded for certain actions, for instance registration or attracting new online casino customers.
Note that no-deposit bonuses are quite rare (except the programs aimed to attract new players). That's why you shouldn't expect that you'll receive tons of gifts just after registering. If some online casinos still offer free bonuses, they are small and will unlikely give you a chance to enjoy games. But when placing minimum bets even $5 free bonus would be enough to play a long session.
Deposit bonuses are much more interesting and diverse. First of all, they can be huge. For example, 300% first deposit bonus is not uncommon, although most casinos offer 100-200% sing up bonus.
These numbers are true primarily for first deposits made right after registration. Casinos online are interested in making a new player transfer as much money as possible and encourage him/her using various tools. Quite often welcome reward consists of three first deposit bonuses allowing players to receive 2-3 times more money than they deposit.
Of course, this generosity has logical explanation. To withdraw reward money players need to meet quite strict requirements. In particular, player should place total amount of bets several times higher than bonus received (suppose you've got $500, then your total amount of bets should be $2,500). In addition, usually some time limitations and game restrictions are applied to clear bonus. Therefore, big reward is often just a bait to attract players.
But after becoming regular player at online casinos you can rely on casino loyalty program which also provides bonuses. Participation in loyalty program itself doesn't require any deposits, but all the benefits this program can give are impossible without a huge number of bets.
The more frequently you win the higher is your status and the more points you receive. Generally points can be exchanged to online casino chips to play or cash out.