You have to learn about the various forms of free play. There are comps that you earn and there are marketing comps. One has little to nothing to do with the other.
I'm going to use Harrahs system as an example, but other casinos will pretty much follow the same general pattern.
You join the players club and the casino starts tracking your slot play.
Harrahs give 1 Reward Credit for every $5 you cycle thru a slot machine.
If you are playing a one coin $1 machine , you are betting $1 a spin and every five spins($5 thru), you earn a Reward Credit( ie RCs). Wins and losses don't come into the formula.
Lets say you play 100 spins. You have earned 20 RCs. Your wins or losses are irrevelant. Lets say you play for an hour and end up with 500 spins. You have earned 100 RCs. 100 RCs equals $1 in comps.
Doesn't sound like much, does it?
Most Harrahs machines offer what is known as Bonus Rewards(BRs). These kick in mysteriously and all I can tell you is as you earn more Reward Creidts in a day, your Bonus Rewards grow at a much bigger pace.
Here is an example. Your first 50 RCs earns you almost no BRs, but at 100 RCs, you'll have earned Brs at about a 1-1 pace. 100 RCs in a day will generally get you around 100BRs, as well.
Now, your play has earned you $2 in comps, rather than $1. Small improvement. But now you start earning BRs at a much faster pace. Your next 100 RCs might earn you 500 BRs, so by doubling your play, you now have earned not $2 in comps, but $2 worth of RCs plus $6 in BRs.
Your comp balance is now $8. Still no big deal.
Now your players club info is forwarded to the Marketing Department. Their job is to get you to return to the casino, so they send you some offers to entice you back. you are pretty much unknown to them so they send you a low ball offer of $15 in free slot play.
Now, your end results of having earned 200 RCs is not the $2 in comps you expected, but $8 in comps plus an additional $15 in free slot play.
In most cases, comps you get from marketing offers will dwarf the comps you actually earn on the machines.