Resignation is a signal that you know your opponent knows how to win so why waste everyone's time playing it out. For high level players you can be confident they know how to win but there might be more than 100 moves left in the game, so not wasting time playing out a losing game is the polite thing.

When playing someone low rated your opponent isn't good enough to think they can win unless there are less than 3 moves left so you may as well just play the rest of it out at that point. Even then, if you are in a simple (rook?) endgame if the low rated player makes a couple right moves you can assume they know the remaining moves so is it worth wasting your time to prove it?