@adamd there are different types, but the most common are federation bots. By following you they pull your toots onto the federated timeline for that instance. In effect they're signal boosters. If you want to keep things local, block away. If you don't care it's best to let them be. You don't need to follow back.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko