Your question is essentially "why do Electron apps exist?" and the full answer would be quite long.

The most important one is that an app's lifecycle can be different than a web browser. You don't always keep a web browser open, but you might want to keep Discord open regardless of what you do with the web browser. That kind of lifecycle management can be tedious and frustrating for a regular user.

Discord's electron app has many features that its web app doesn't such as "Minimize to system tray", "Run at startup", "Game/media detection", "In-game overlays" etc.

Even PWAs can't have most of these features, so that's why we have to deploy an entire browser suite per app nowadays.