I went to them 25 years ago as well. I get the same thing from the organized promoted ones as I got from the old ones. I didn't need it to be "underground". I just needed the people, the music, the vibe.
There were events where I met so many people that it had a snowball effect. In addition to anyone there I already knew, I'd be building/merging new friend groups on the fly and sometimes hours would go by before I'd see any of the people I came with.
Conversely, at sanctioned events I'd stick tight with my friends pretty much all night... sure you'd meet other people, but not nearly to the same level. Generally dance more and socialize less.