The value added for Nostr is that all the apps agree, more or less, on the underlying data structure. So a standalone notification app makes sense. It lets you preview and prioritize incoming notifications, then open and interact with the content in whatever specialized app is the best fit.
Non-Nostr apps can't really do that. At best you have to wrangle a bunch of different logins and unique APIs for all of your apps.
It's like the Windows notification center, but you can take it anywhere (especially if you can access the notifications tool as a web or mobile dashboard).