I didn't stop using Mumble or IRC because they couldn't, combined, meet my needs. I even prefer them to Discord on a principle basis.
But everyone I wanted to communicate with moved to Discord because it was easier -- as it had both core features which are necessary for gamers.
That's the real reason why you use Discord. Peer pressure.
Discord is in no way a replacement for a forum
I’m quite a techy person and I even wrote my own frontend to Discord a while ago, but I’ve struggled with things like bouncers and authentication to the point where it’s just easier to set up a bridge to Discord and use that when I need to use IRC, and then I miss features like editing and profile pictures (there are proposals to add these things to IRCv3 but they aren’t widely supported)
IRCCloud seems to have a good solution to some of this but is paid
It is, however, a potential alternative to one common use case for Discord, namely: "What we really wanted was a web forum, but this is easier."
This describes all 15 of the Discord servers I'm currently on.
What does discord offer that reddit doesn't? (In terms of being a web forum)
(My only experience of Reddit is from Boogle searches landing there [an immediate point in its favour over Discord] and it appears to be pretty heavyweight, loads down the browser even more than Discord, and has ads everywhere - maybe the experience is better if you actually create an account and log in?)
Discord's initial pull was from gamers who were using teamspeak/ventrilo/mumble + twitch chat and maybe IRC.
If it doesn't do VoIP then it's not a viable alternative to Discord.
It'd be like if LibreOffice couldn't save new files or if Firefox couldn't render images or video.