The LARPer I know (one, singular.) Says they do it for mainly to spend a weekend in a field getting drunk on mead dressed in medieval garb, with their group/ team/ clan/ whatever their collective calls themselves. But she says that it's a great laugh. And they do campaign battles along a story. It all sounded very Warhammer meets D&D without miniatures.
I'm sure it's a good laugh, and I'd never knock anyone for it, and I'm sure if I tried it'd be a laugh, but I much prefer to sit around with a few mates and have a round or two of diplomacy.
EDIT- though I suppose in a way, anyone who daydreams fantastical things from time to time would probably enjoy the chance to have a shared collective 'daydream' with a group of other likeminded people, and without such silly things as monthly fees, pay-to-win armour and swords, and the knowledge that if a server goes down, you can still keep playing. I mean, think about it, who hasn't daydreamed about being either a wizard, pirate, Robin Hood, ninja master or something from time to time...