It encourages people to speak rather than type.

You don't get hundreds of messages per day by writing in email style. You get there if you have a lot of synchronous or near-synchronous communication in chat. It's kind of obvious that voice call suits this better, but there is friction involved in making an actual call.

Discord voice channel might reduce the friction if you make this culture of hopping in and out of it.

If you don't care about Security/Privacy, Discord has a great UI and is probably significantly better for free-flowing group conversations

Discord was originally a place to organize for playing videogames with people. If you're using it for that purpose, you're probably on your computer already (because that's where the games are). Thus, the mobile app ends up being more for checking in with your community while away from the computer, rather than a primary driver of your engagement with the platform.

They could have been using it in browser. One of discords underrated decisions is that the browser version is fully featured and doesn't force you to download the app, a new user can join a server in their browser with just a name which is as low friction as you can get.

I do think that whatsapp could do this tbh if it would actually provide its api like discord does.

Telegram feels like it can definitely do such stuff and I found in my opinion that its way easier to host telegram bot on cloudflare than it is on discord so theoretically you could even have it as a cf worker with a deploy on cf button so as to even people who don't know too much about deploying could use it.

WhatsApp has this now - https://faq.whatsapp.com/1973730693032338

WhatsApp should totally add that as a feature! A voice chat room with join notifications.