Yes, free spins that you get immediately after registration without depositing are really free! The profit you earn with it will be added to your account as bonus money.
If you can play long enough with this bonus money, you can pay out the winnings. How long you have to play differs per casino. Read the bonus T&C’s and look for ‘wager’, this is the number of times you have to bet the bonus money before you can withdraw it.
An example: You win €10 with no deposit free spins and you have read in the bonus conditions that the wager is 30x. Then you have to bet a total of €300. If you have succeeded, you can withdraw the money, including the money you have won along the way.
Casinos often have additional conditions, so make sure you read them carefully. For example no bets over €5. Maximum wins. Or certain games only count for 10%.