You must wager bonus bets to turn them into cash. Then you'll keep any profit from those bets, but not the bonus bet itself. So if you put a $50 bonus bet on the Bucs to beat the 49ers at +400, you'd earn $200 in cash if the Bucs win ($50 x 4.0). If the Bucs lose, the bonus bet just goes away.
You can't withdraw bonus bets or bet credits, but you can withdraw winnings. Bet credits and bonus bets cannot be withdrawn, but they can be used to wager on other sporting markets your sportsbook has to offer.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.