Q: Why are all these bots following me?
A: Mastodon and GNU Social are /federated/ network with many individual servers. Only toots from profiles followed by someone on a given server propogate. "Follow Bots" ensure that more content is spread over more of the network. This may or may not be a good thing. And other bots may have other ideas. I'm not settled on the question myself.
A con argument: https://mastodon.social/users/pan/updates/1697694