If Tumblr were to add ActivityPub support, it means users on Mastodon could follow Tumblr users’ posts from their own Mastodon instance — without having to use the Tumblr app. It could also provide Tumblr users with an entry point into the so-called fediverse without having to face some of the complexities that are involved with signing up for Mastodon for the first time.
Good for them, but we’ll see what the effect of this actually is.
It occurred to me that I don’t visit tumblr today, I might continue to not visit tumblr tomorrow, so this might squarely land in the “not really affecting me” camp.