The quick answer: Credits are unrevealed payouts.
The long answer: When you buy credits at b spot, you're placing bets on live horse races. Once your wager is confirmed, the funds are wagered. Your credits represent the money you placed on the race.
After the race results are finalized, you use yours credits to reveal your payouts while playing games. All your payouts then go directly into your cash account.
Remember, you cannot make a withdrawal from your credit balance. Credits have no monetary value.