I probably don't need to explain why fake accounts and bots are bad, but I'm going to anyway: spam and scams are the biggest threat they pose.
They also make it hard to determine accurate metrics and as much as I hate to say it, metrics are important if your account is part of your business.
Thankfully, there's no algorithm here, so a bunch of bot followers aren't going to consign posts to oblivion by not interacting, but they still take the fun out of a platform really fast.