For the minimalists of us. There really is no reason to constantly take up that real estate.
When Safari introduced this feature I was stoked. When I learned about workspaces though, I immediately switched and began searching for a browser that looked and worked great. Aside from this feature Opera nails it with its great sidebar (in autohide) and great tab grouping.
I know I'm not the only user who doesn't need to see the address bar all the time, and implementing this feature seems like it'd be relatively simple. Vivaldi does this beautifully and I'd stay on that browser but it's proving to be too laggy.
Keyboard commands can easily open the address bar up for the moments where we need to access an extension's settings, and the other 99.5% of the time that it's serving zero purpose we get the extra space.