It's a building block to make E2EE possible at Fediverse scale.

I've written about this topic pretty extensively: https://soatok.blog/category/technology/open-source/fediverse-e2ee-project/

If you can build in Federated Key Transparency, it's much easier to reason about "how do I know this public key actually belongs to my friend?" which in turn makes it much easier to get people onboarded with E2EE without major risks.