When a free bet wins, your sportsbook will only give you the profit from the wager and not the stake. This is known in the industry as a Stake Not Returned (SNR) free bet. Whereas your successful real money bets will see your profit and stake returned to your account.
The main reason for the free spins no-deposit bonuses popularity is the low risk of losing bankroll while playing your favorite games. Newly registered players benefit from bonus spins on sign up without a deposit since it is a chance to try out a casino risk-free and win money.