I'm not saying that everyone who seeks high karma scores is like this, but astroturfing PR/advertising firms will pay real hard cash for accounts with an established history of trust from the community. I've read many stories from people who have gotten private messages from such places with an offer to buy their account. Or perhaps, like gallowboob, you want to parlay it into an actual social media gig (UNILAD for him).

I do believe for most people though it's just a sort of fame-seeking thing, for whatever reddit 'fame' is worth.