Approximately 30% of the road miles in the U.S. are unpaved. In some parts of the country, having a gravel bike greatly expands the options for cycling. I don't know what it's like now, but when I lived in northern Wisc. in the 70's the highways were paved, and about 50% of the roads in town were paved, but most of the rest were gravel. The coast highway to Duluth/Superior, for example, was paved but in the winter when storms came off the lake we were sometimes forced to take an inland route through the national forest, all of those roads were gravel. There were thousands of miles of logging roads that were gravel or packed dirt.
Times Square is less than a 45 min drive away from where I live now (no traffic), but within a 1.5 hour ride I can find miles of gravel roads in upper Westchester and Putnam counties. Admittedly, near where I live you have to go looking for gravel roads, but that is not true in large parts of the U.S. I don't really want a gravel bike for where I live now, but in large parts of the mid-West, South, and particularly in the West, gravel roads are very common.
There are lots of packed gravel rail trails within easy driving distance of where I live - for example the D&L trail in Pennsylvania - and others in NY, for example the Erie Canal towpath - that offer hundreds of miles of packed gravel riding. My wife and I managed to handle the D&L canal on our tandem with 32mm road tires, but a gravel bike with wider/softer tires with a tread would have made the ride a little easier, particularly with all of the fallen leaves on the ground.
I'm not sure what Canada is like, I imagine the proportion of gravel roads is higher there. In Europe, I have no idea about the opportunities for gravel riding. As for Asia and Africa, and Australia, I imagine they have many hundreds of miles of gravel roads in some areas.