Although the amount of money you get depends on the site, you can always avail of some of the offered freebies.
Online casinos use a jackpot system in place of pay-outs. Instead of paying for a single spin, you can win a whole pot for spinning as many times as you want. The catch, of course, is that you need to be lucky enough to win such a huge sum of money. The casino bonuses offered by most of these sites are designed to increase your chances of hitting it big. Since slot machines are used to dispense winning entries to video gaming sites, they are often places affiliates of gambling organizations. As such, they receive a cut (or in some cases a large part) of the total winnings.
Slots that use reels have a fixed number of spins until they fall off the reel. Like with slot machines, some sites offer double the number of spins for each bonus paid out.