There are different kinds of bonuses offered by online playing sites since bonuses are one way in which sites compete with one another for customers.
The purpose of the bonuses is to attract new customers and retain existing customers. A player that sees a good bonus offer reads the terms and finds it fair and better than the deal he is receiving at his regular playing site, might just join the new playing site.
This is the purpose of the bonus and what the site offering the bonus wants to see happen. Bonuses can be devised in different ways. They can be no deposit bonuses where the player receives a certain amount in time, money or playing units to allow the player to tryout the site and the software. Other bonuses may be tied to the first or first several deposits where the site matches a percentage of the deposit.