Make no mistake about it, all casino sites and venues want you to play their slot machines, for each slot does of course come with a built in and pre-set long term average pay-out percentage, so the casino operators are going to earn a percentage of each players stakes over the long term.
With many slot games offering pay-out percentages of around the 96%, that means over the long term a casino operator will earn 4.00 for every 100.00 played through each slot machine they have on offer, so the onus is on the operator to keep you playing to increase their theoretical profits.
As such many casinos will have a players club or a casino comp club as they are known, and what they have been designed to offer players is a way of earning additional playing credits or lots of other extras based solely on the volume of wagers players play through on slot machines and other casino games.
Each time you play through a slot machine a certain number of credits, irrespective of whether you win or lose you will be awarded with comp points, and you will then, in the case of an online casino be allowed to exchange those points for playing credits once you have amassed enough of them.
Most casinos have an automatic system in place that allows players to redeem their awarded comp points and turn them into playing credits instantly, but the number of points needed to earn additional playing credits can and will vary from casino to casino.
What you should therefore be on the lookout for are casinos that award the most comp points and have the very lowest redemption rates too, as you will earn more points which can be turned into a higher number of playing credits when you play slot machines at those casinos.