I haven’t really used it, so this is all second-hand:

As far as I understand, it’s a younger project with more papercuts than Lemmy, but more features. Instead of just being a link aggregator like Reddit/Lemmy, it’s also got microblog functionality so it plays better with Mastodon. For users of Mastodon, having a one-stop-shop for both services is pretty handy.

It’s much harder and more problematic to set up kbin instances, so almost everyone just uses kbin.social. This means it’s a very centralized fediverse platform compared to Lemmy, but that might be seen as an advantage to some.