Fire roads are easy mode on an ADV bike. They can do double track quite well too. They start to struggle in mud, sand and single track.

I'd presume that pure street motorcycles would fair pretty badly on even double track.

So yeah, ADV would seem to be useful for something other than wanting to look rugged.

This is exactly right. The contrast that piqued my interest was something like knobby tires on a heavy bike that spent its life on the street. Turns out people value comfort (here, seating position and long suspension travel), but those things only seem to sell with a rugged aesthetic. Balancing these needs is something of a Hard Problem, so it's interesting to watch this play out throughout the industry.

Totally, in London i regularly encounter cobbled streets, my big tyre gravel bike is perfect.