1) discovery
If I can’t find a forum, it’s not much use to me!
2) single sign-on
I don’t want to make a million accounts for a million separate forums. I want one account that I can take with me to every forum.
Both of these issues were solved by Reddit. This was its major value-add. The user base brought all of the remaining value with them.
This is also the value of the fediverse, of which lemmy is a fine example. Yes, the technology is way more complicated than phpBB, but it solves these very challenging problems which present huge barriers to growing an individual forum (which must be overcome once again, every time for every forum).
This is akin to online dating. Nobody needed 5 dates a week 20 years ago and because it's an option now doesn't mean it was a problem than. Optionality is just a byproduct of social platforms.
If you're referring to the time when the "old forums" were new, that is a world which no longer exists: a world where Google search results were useful and not overloaded with paywalled sites and SEO spam. Today, you're going to have a very difficult time finding those "old forums" unless you know the exact name of what you're looking for. And forget about browsing.
As for "people today have too many options, back in the day we had fewer options and we were fine!" that's a very old argument you'll have a hard time convincing many people of.
Why do you want that? Why would you want your identity on a functional programming forum to be the same as on a Star Trek fans site and a furries meetup group?
The other alternative where you don't care if functional programming and Rust programming forums are on the same id is the issue without the option of a single account