You can think of it as an API. If your API is called, should it work differently depending on whether the request came from a browser or from a terminal curl? Probably not.

Similarly, if you have a message that, calls for your model to be changed (for instance, adding a product to the cart). That message describes cart logic. The product could be added directly from the product page, or the frontpage, or from a suggestion in another product page. It is probably not relevant to the cart how exactly the product came to be added.

Not all the model is like that. There is a subset of the model which exists only to describe the state of the view. But, usually, a significant part of the model is dedicated to page-independent data.

> I want to be able to look at a small part of the app

In my app, the small part of the app I'm going to look at is a single feature which may be used in multiple pages. It is easier for me to keep a consistent behaviour if that feature is independent from the view.