However, one might have albums in the library for purposes of documentation. Because if they are in the library you can see them in Focus, when otherwise you wouldn’t. E.g., if you want to look at a history of rock music in the 70ies, then you can see them there. I, for one, do this with historically significant albums even though I might never listen to them again.
It’s a result of Roon’s two-class system for in-library vs not-library albums.