An "open" data format they can use to get sites to integrate with their servers without signing up for an OAuth token or anything like that.
Facebook Connect used to be a big deal.
Major sites once let you comment on articles via Facebook only.
ActivityPub will let them pull that off again without the backlash of the past (no more "walled gardens").
I've already seen a few blogs that accept ActivityPub replies as comments.
Threads offering 130 million monthly active users puts Meta at the #1 spot for those comment sections now.
They could effectively ignore Mastodon, Friendica and the others and dominate by helping ActivityPub go more mainstream without making any changes or extensions to the spec.
Think about it, if ActivityPub becomes the HTML of the social web like the Mastodon community and #Fediverse as a whole dreamed of, Threads wins.
liaizon