Many casinos give you free play when you sign up for their Players Card- I always visit the Players Club desk in a casino for the first time and inquire- some have you spin a wheel for free play, free buffets etc- doesn't take much time to ask what their current promotion is , if any..
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However, these free play credits usually come with terms and conditions that need to be met before any winnings can be withdrawn as real money.