Discord isn't just "chat text", it's also "real time discussion". That's why for example you see indie game devs often creating Discord channels, to contact their userbases.

In this sense, it partially overlaps with a forum+link aggregator like Lemmy, that allows deeper and asynchronous discussion. But they're still distinct enough so both a hypothetical "Fediscord" and Lemmy would benefit from federation:

• If discussion in a "Fediscord" server is getting a bit too deep for the medium, you can redirect people to a Lemmy thread.
• Conversely, if people just want to chitchat, in Lemmy they'll end creating noise. They can simply do it instead in a "Fediverse" server.
• A Lemmy mod team meeting in "Fediscord" to decide how they're going to moderate their Lemmy comm.

Nota bene: privacy is a concern and federating all content would be certainly unwise. Even across "Fediscord" servers. But some content is sensible, as long as you have a clear division between "public" and "private" rooms.

Similarly, why would I want to view a chatroom through Lemmy? Why wouldn’t I just want to go to that community through it is app or site or whatever?

CBA (can't be arsed) is one of the main features of the race that we belong to, humankind. If you're interested on a single "Fediscord" chatroom but can't be arsed with the rest, you could follow it through Lemmy instead - it'll be less optimal (F5, F5 all the time to see if there are new messages), but still useful.

It just feels a bit like blockchain all over again. Federation and activitypub are great tools for some purposes but people seem to want to use them for everything.

I get that when you have a hammer everything looks like a nail, but that isn't my case here - I genuinely think that the "Fediverse forums" (Lemmy, Kbin/Mbin, and any potential newcomer) are in a central position within the Fediverse, and should capitalise on it by federating as much as reasonably possible with other Fediverse services.