Because it's fun, and it often garners an entertaining reaction from players so insane and obsessed with their game that they pretty much deserve it. Let me give you a good example.
Some time early last year, a friend of mine was telling a bunch of us about how a guy we knew was a fussy, whiney "it's all mine and you can't play with any of it" player on a Minecraft server run by a friend of his (friend of the guy who told us, not of mr whiney-poo). Some examples of this guy playing in a way that was ruining the game on that server far more than griefing ever would included:
*Building a wooden wall around an
enormous area and basically declaring that he was not to be bothered by such petty things as other players in a multiplayer game where sharing resources and living together is encouraged. If he was, you'd be chased out with a sword
*Basically having it so that anything he saw was immediately his possession (In a cave full of plentiful spiders, he killed one and my friend accidentally picked up the web. He wouldn't stop bitching at him to give it to him, when he could've easily just gotten some more)
*Having OCD about his house and possessions to the point where he tried to kill my friend for taking one torch from his house (admittedly my friend was just trying to provoke him after all the shit he'd already done, but it's one friggin' torch)
Upon hearing this, we immediately began planning "Operation: Troll the wranga" (hey, nothing against
orange hair, but a silly operation designed to piss somebody with orange hair off needed such a name). We planned, in conjuction with the admin, to go into the game together at the same time dressed in a troll-face skin (what?) and, from admin powers, burn his precious little complex down with lava and TNT, then post pictures of the destruction to the guy's facebook. It was lulzy, and by Christ was he pissed.
But on the flip-side, when it's not used for any sort of just or justified reason it can be a bit annoying (but not to the point where I'd be so butt-hurt that I'd, say, start a thread lashing out at griefers, or do just that by thinly disguising it by saying that "I don't understand griefing"). After the operation we began playing Minecraft together alot more, always setting up servers and playing together when we had the time. On one of thee servers we lived peacefully together in a village in a house we'd built together, sharing resources and having a blast. At one point (and I think it was over a matter of some glass that was accidentally taken out of a furnace and used) one of my friends got really peeved, built a different house to sulk in, and then after a while of mutually shared jabs at each other about being a sulk and "stealing" glass this jaded friend went and burned the house down that we all shared. That was annoying because it was three hours of, dare-I-say, work and cooperation, all lost over such a silly matter.
In the end, after he realised what he'd done he ended up apologising profusely later over Facebook, and we forgave him. We played without a hitch onwards after that.
