Free spin bonuses are bonuses offered are a tactic that casinos use to lure you into playing their games in hopes that you will add money to your virtual wallet to continue playing.
Free spins typically come with a catch. Either you have to make an initial deposit in which they match a percentage of and give free spins or they offer free spins but you have a bet multiplier you have to reach to cash out.
That means that if you get a offer for $20 in free spins you have to bet a percentage that's pre determined by the casino. Generally that percentage is very high, I've seen them at 400% even. On a $20 free spin at 400% means you have to wager $8,000 before you could cash out your winnings on the $20.