My opinion is it’s just the data, Meta is all about collecting data. Being part of the fediverse means they potentially get access to a bunch of data through scraping and user interactions.
For example someone might not follow a gardening account on instagram/Facebook but they might join a gardening community. That’s valuable data.
They can also boast about the new technology. Maybe they hope it will revive the meta verse lol
That’s a very good point, I had forgotten that. Anomander has a good perspective on this too in a different reply