A really simple thing is that sometimes I really don't need the website for more than 10 minutes a year, but dealing with turning off notifications, being forced to create an account, being asked about app updates, having it on my home screen, etc is not worth it.

I think some apps feel nicer than a website ever could (because the app can take advantage of stuff like force touches better, and offer a nice custom usage of gestures). But some other places it really just feels like the app exists to push notifications...