Now imagine you want parts of your app to be SPA and some other parts of your app to be SSR. This becomes super complex. Frameworks like Reframe allow you to do that. (I'm Reframe's author (https://github.com/reframejs/reframe).)
Admittedly, mixing SPA and SSR views in one app is uncommon today but that's because people are not aware that they can do that. This will change.