Probably to use the "Copy Value" buttons? Is there malicious suspicion or some other reason this is interesting?

It's not a tradeoff that is worth it to me.

I don't want a Facebook extension to have access to arbitrary data that I copy and paste in my browser, which may be unrelated to the app I'm developing at the time.

From what I can tell, the extension doesn't even have permission to make http requests so they aren't transmitting your data anywhere. (They also explicitly stated they don't on their listing).