Casinos often offer free play as a promotional tool to attract players and provide them with a chance to try out their games. However, these free play credits usually come with terms and conditions that need to be met before any winnings can be withdrawn as real money.
Why do casinos want you to use a player's card? Because we live in a world now where data is valuable. By having our guests play with a card we are able to track their play, what machines they like, how much they bet, how often they visit, how far away are they coming from, and much more.