It's because Discord provides a functionality Lemmy does not yet offer. Forums having a connected chatroom is not a new thing at all, and that was true with subreddits and discords. Back when I moderated forums, the mod teams had a private mod forum, but also hung out in the IRC chat. Easy, instant communication for the community, particularly it's management, is useful.

The problem isn't the existence of a chatroom, it's that they used Discord. But I don't believe there's a fediverse alternative for chat functionality like that, at least not yet. So it's a matter of which centralized platform you choose: Discord, Matrix, etc

But Discord is also a useful reference point for new users who may need guidance on how to use lemmy, but already know how to use discord. It's familiar, and popular. Call it onboarding if you like. It's using Discord's platform to help ease users into Lemmy and the fediverse as a whole.