A heads up display doesn't need a 3D rendering of the environment around it because the environment is already visible through the screen. The 3D rendering is so you can see what to expect before you get there. If you don't understand why that could possibly be useful then I don't know what to tell you; you'll have to take it for granted that some people's brains work differently than yours and can benefit from seeing places they are about to visit in 3D before they get there.

Apologies, I meant my point to be that navigating a place is more helped with AR techniques than it is VR ones. Which, as you say here, is less helped by 3D rendering than it is other things. Indeed, I meant that to be my point.

Do I think it could be useful if you rehearse navigating a place before getting there? Yeah. Ish. I can see obvious military style value adds for that. Average person, though? I still have a hard time seeing the value.

Reminds me when places were offering video tours of places. Is a neat idea. But ridiculously low in actual value.