Hey, bit late to this discussion (found it while searching for something) but since you seemed interested in a casual user's pov:
I'm a member of quite a few Lemmy communities that are really small, and I'm very active on Mastodon. So having those small communities in a list feed on Mastodon is really handy since I don't miss anything and can just jump in with a reply on stuff without switching over to Lemmy.
I also post a lot of the same type of stuff to both platforms and sometimes it makes sense to keep that separate, but sometimes with niche interests it's nice to be able to cross-post and get both groups of people chatting together in the comments.
Of course this is a moot point because federation between my Masto instance and Lemmy is currently broken, but it was really great before and I miss it a ton.