Some games are daft.
Some games have silly heroes, and even sillier monsters to fight. Blast Squidzilla with your Pew-Pew-Laser-wielding three-armed Cleric until you reach level 10, or die trying!
Some games encourage you to backstab all of your friends, make silly jokes, puns, run like a coward, and sometimes even cheat.
That's your basic Munchkin. It's all gravy if you can get away with it!