The ability to trade
Betfair gives you the ability to trade. Trading is trying to lower your risk and ensure a profit, irrespective of the final result. Have you ever backed a football team who take the lead, but are looking like they might concede an equaliser?
You could sell some of it back and walk away with a guaranteed profit. Now you can lay selections as well, so you could choose to recover your stake to leave you with a 'bet to nothing' (a bet where you break even on your worst outcome), or hedge your position to give you a profit no matter what the result.
Betfair is more than just a betting shop where everyone is focused on picking the winner of the race. Because Betfair was modelled on a stock exchange, financial traders have recognised that they can trade on racing and sports markets just like they were foreign exchange markets. And even better, there's usually a race every 10 minutes during the afternoon so profits can quickly be invested.