My idea is to provide an API for web developer to easily control how the previous and forward button of the browser works, make it easier than using history API or developed with spa framework, to enable some non-spa site can also make more natively user experience or working like in the app.
What's with History API? it already allows adding custom entries with states invisible to user and changing url if that's needed without reloading the page (pushState). And the rest is handled by browser. Changing that would mean to change the HTML specification (or break compliance) and possibly user expectations about working with tab history.
I guess some js lib can already make working with history easier without the need to change DOM API.
history API is more like a passive way to make use of the previous and forward button (you listen to the change of the URL from the user's click of the button), but as a web engineer, sometimes i image a more straightforward way (like direct listening to the events from the toolbar button, not the change of the URL)