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.
Out of 24 discord servers I am on, only 4 are related to games.
Most are software related, or why not genealogy or mesh networking?
I never started using discord for gaming, I just find its everywhere.
Gaming was the original use-case because historically every gaming clan had a teamspeak server, a forum, and an IM group chat, each of which was a pain to set up and administer. But it just so happens that this is also a pretty good set of features for basically any kind of small-ish online community, so it's taken off across a huge number of areas (my hot take is that feature wise it's actually better than Teams/Slack/etc, especially in terms of how the voice call UX works, and inertia and general reputation/theming are the main reasons it hasn't taken off for internal company communications).
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.
Its main draw isn't explicitly gaming, but where your social interactions more often align with that of a friend group / in a group chat dynamic where everyone's talking with everyone or hangs out together at similar times. Gaming is just how it started, because nearly all gaming groups have this sort of social dynamic. (Obviously Discord does support DMs)
The fact that it also scales up to handle hundreds or thousands of concurrent users made it work for pretty much everyone. Suddenly you had one place you could go where there were thousands of other people all interested in the same thing you were interested in, and could meet new people or new friend groups over that said shared interest.
It also absolutely needed:
- persistent, non-disappearing text chats that last forever*
- Free image uploads (and thus meme sharing)
- voice chat & screen sharing, proxied (so no DDOS attacks after a malicious person grabs your IP in a public voice chat)
- role-based access control, so you could have an announcements chat for your server where only admins can say things, you can appoint certain people as moderators, etc
- a mobile app with push notifications (to mimic the text message paradigm of being always available to chat with your friends)
- Discord building out the ability to connect custom bots so early
*: No word yet on if they'll ever start cleaning up old messages and attachments. There must be hundreds of petabytes of attachments stored in multi-AZ google cloud buckets that will never be accessed ever again.