Bonuses vary from site to site. Gaining extra help from promotional offers have only real benefit if the terms and conditions of the offers are favorable to the user claiming it. Some bonuses are good, some not so good. You have to weigh the options up and you may discover that the welcome bonuses, though larger in size, are not as beneficial as the casino's more regular offers that are presented to players throughout their ongoing time as a member.
Claiming rules mostly require players to make a deposit first or series of deposits. Turn to the terms and conditions for validation of the process relating to the bonus you're interested in.