I've actually thought about this myself recently. Naturally, users tend to pay attention when they see a post or comment from a high rep user. I guess we assume that this person must be a fountain of valuable content because users with higher reps than them have voted them into this position over time.

Bots take that away entirely because there's no quality filter. You pay the price, you get the upvote and increased rep that goes along with it. If you're buying votes on every post you make, this can soon add up, and it kind of ruins the rep system altogether.