Most places won’t allow you to withdraw your deposit and earn a bonus until you’ve bet a certain amount on their site. That amount varies but usually requires your deposit and your bonus to be bet (or ‘rolled over’) a total of 3-10 times!)
At Betfair, all you need to do is deposit $50 and if you win, you keep the full amount! There are no further betting requirements.
If you happen to lose your $50 bet, Betfair will put the money back into your account! From there, you may choose to withdraw the money at any time you wish.