The idea is to maximize screen real-estate. With this feature, the address bar would only appear on hover or after pressing a keyboard shortcut. This is similar to Arc's implementation, except that the address bar is at the top.
I know focus mode already exists, but it doesn't let you see the address bar again unless you exit focus mode.
Here is how Arc does it:
Hidden:
On hover:
Ideally, Orion would do something similar, albeit with the native-looking safari-like address bar.