You know, like Wells Fargo was reportedly trying to foreclose on houses they didn't even own mortgages on, and signing up consumers for loans without their consent or knowledge...
My bet would be they used them for as many purposes as possible. It would seem to me that a prime use would be bilking advertisers. If FB was selling you X number of eyes on your ad, and most of them were fake accounts, you’d be paying for ad hits you never really got.
Another would be to create fake “statistics”. X% of Americans favor gun bans. X% of Americans favor impeachment. Things like that.
I can’t think of other uses at the moment, but I’m sure there are many.