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).