Generally, slot/video poker free play credits convert to real money after being played once - i.e. you'll getR$10 slot credit, then you playR$10 worth of spins, any winnings are now real  cash.

When a player receives "free play," they are more likely to stay in the casino and use the credit to play additional games, which increases the casino's revenue. Additionally, "free play" is a way for the casino to control the amount of money that is spent within their establishment.