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.
Bet credits and bonus bets cannot be withdrawn, but they can be used to wager on other sporting markets your sportsbook has to offer. Some sportsbooks make you use your bonus funds or bet credits in one lump sum.