You can recharge for postpaid numbers too. It goes into account balance and can be used for things like ringtones, or in case you switch back to a prepaid plan, or for paying for someone else's recharge. It takes the money from your account balance and you don't have to enter your payment details.

It's usually called an account topup, where you just increase your balance so that you can use it later when you need. I do it a lot.