Thanks to Lemmy, people can stop designing forum websites from the ground up
Think about it: Lemmy provides you with a ready-made frontend and backend --- all you have to do is host your own instance of it. The following could all have been implemented as Lemmy instances, had it existed at the time:
What makes you think everyone was doing it from the ground up? vBulletin, and phpBB were both released in 2000, and Simple Machines was released in 2001. If a forum was custom, it was for a reason.
That's not really any different from existing forum software, though, and they have their own needs and things.
Forums existed well before Lemmy, and the reason that places like Reddit, and Hacker News made one of their own, instead of repurposing something like phpBB, is because they had special needs that the default forum software suite didn't cover, or it didn't suit them.
One of the strengths of Lemmy is its integration with the Fediverse, and how different instances can community with each other through ActivityPub, but that's not necessarily something that you might want on a forum. More users can be nice, but it can also bring along with it its own moderation challenges and things, and sometimes you don't want your support forum to be clogged up by idle chatter.
You also have places, like Beehaw, that might have more stringent account creation restrictions, and you don't want some random user over on Reddit, or wordpress to just pop over, bypassing that.
But the idea of having just one account for everything isn't new, or unique to Lemmy. Facebook, Apple, and Google had the same idea (hence the "sign in with X" feature), and there was an open-source equivalent called OpenID. It didn't get particularly widely implemented, but it still exists for people to do the same. In theory, with an OpenID, you could just use it for everything, without having to register for a separate account, and all of that excitement.