Here's a story that illustrates why I favour federating with Chains ("Threads"), even though I'm opposed to Meta's very existence.
Alice and Bob follow each other on InstaGrope. Alice, being more of an early adopter, signs up for Chains, and convinces Bob to do the same. Then Chains turns on AP federation. Alice realises she can still follow and talk to all the same people on Chains from other fediverse servers, and moves to one.