The way my orthodontist explained it was that over time, our mouths have evolved to be smaller yet we still have the same amount of teeth. He said that if you're one of those lucky people whose jaws are big enough to facilitate four extra molars, you don't have to worry about it; if not, cramped wisdom teeth can cause impactions, come in sideways, crowd the rest of your teeth to the point that they become crooked and "tombstoned" (like lopsided headstones in a very old, untended graveyard).

Then he proceeded to remove my molars and let my wisdoms grow in and replace them, so maybe he was just nuts, I dunno.