A postnuptial agreement, signed after your wedding day, is similar to a prenuptial agreement. It sets aside certain assets and/or addresses pressing property division questions. If you decide to get divorced, you already have that agreement in place.
The need for a prenup is clear: If you worry about divorce, you want to protect yourself financially in advance. It's your way of saying that you want to get married, but you understand that doing so puts you into a legal contract with someone else. You want some way to spell out the terms of that contract to protect your own assets if the marriage doesn't work out.