Meta is categorically evil, but the pretty obvious gain from federation is the same as it is federating with anything else: content. Threads has people posting on it, some of those people say interesting things … the end.
That’s not to say that outweighs the downsides, like some of that content will also undoubtedly be hateful bigoted trash, and the moderation load of suddenly dealing with an order of magnitude more posts will be a huge strain on fediverse admins who choose to federate, but there’s undeniably something to gain.
There are literally, not exaggerating, over one billion Instagram accounts in existence. It’s self-evidently not the case that they have just silently registered everybody a Threads account and are counting those numbers.