The bonus given is usually in the form of free spins, not a balance credited to the account. The number of free spins is also limited, and you may only win a few $ that you cannot withdraw. Usually, those who give such bonuses are new casinos with the aim of attracting new users to try their games.
The bonus given can be used, but I do not know how lucky you get in the free spins. After you try the game at a new casino, you will usually make a deposit if you are interested in continuing the game. New casinos also usually give first deposit bonuses.