The root page
The root page of websites used a special root-page app, that allowed you to drop any page inside its main bucket, and it would display it.
The reason for this approach was to allow using components even for your root page. Also, you can't use drag-and-drop to replace your root file, as it's fixed in place. So this trick allowed you to move its child instead.
Even without components, if page apps came with built-in layouts, you would need the ability to somehow replace them. So this solution worked for that as well.
So it worked, but it was a weird edge case that worked differently from all other pages. It needed extra learning. Was just out of place.
The new page app solves the issue differently. Your website's root file is a page like all the other pages (no special root-page app), and it does nothing but provide a container. So now there is no reason to replace it, simply replace the content on it.
The only caveat is with components: now you can't easily use a component as your root page. Which is okay. The main purpose of components is to avoid repetitive work. Having to update 1000 pages individually would clearly be a huge pain. Only needing to update the component, in 1 place, is obviously better. Now you might need to update it in 2 places, if it also affects the root page. That's not the end of the world. Of course, It's very common that the root page of your project has a unique layout, in which case you wouldn't have a component for it anyway.