Because nobody likes playing with cheaters. (thats preposterous!)
Playing games should be fun.(dur) In order to ensure the best possible online multiplayer experience, Valve developed a state-of-the-art anti-cheat system that automatically detects and permanently bans any player using cheats or hacks on a VAC secured server. What this means to you as a player is that you don't have to worry about annoying cheaters getting an unfair advantage and disrupting your game.