I think you could probably just say "it became popular because it's free and everyone could use it to game with friends". Those of us that have fallen out of gaming or people that don't game might not literally have any idea why anyone uses it. So you could both be right. You are taking their zero to one by providing the answer.

I think there is a whole generation of people that have very fond memories of discord and everyone outside that window does not have that connection at all.

An addendum to that might be that other people have used other services before discord even existed that had better (?) feature sets but didn't win out and that can create confusion because neither group of people understands what the other group is talking about because there is no shared experience.

There are discord communities around hobbies and shared interests that have nothing to do with gaming and have thousands of users.

But half the good is that they are not some wide-open, forever-indexed forum like reddit.