This gets into territory of things that are verboten on this list, but I will say that I have long cherished the archival videos that made it into YouTube. I think in particular, the work of Michael Bennet was preserved digitally through YouTube in a way that it hasn’t been by living theater - particularly when we compare his work to that of Fosse, which has a whole industry keeping a version of it alive. YouTube was how I came to understand the glory of “A Chorus Line”, and also how I was able to witness the genius (and insanity) of Buddy’s original dance in “The Right Girl” - something that I can’t imagine ever being performed again, just for the sheer danger of it.
We often get into debates about theater as a live form versus a filmed/archived form (a debate that always seems to exclude cast recordings from the argument), but the real downside of our art form is how poorly it’s captured to be studied and appreciated by those who weren’t alive at the time the work was made.
To get to the rest of your question - I think YouTube has certainly been instrumental to the fandom. I don’t think we would have Broadway Con without all of the YouTube videos of people posting commentary, sharing self-tapes of songs, etc.