In my area there are many miles of trails paved with crushed limestone. This surface is mostly smooth and mostly free from larger gravel. These trails are unlikely to be paved anytime soon.
My road bikes are fine on the MUP paved trails and on lighter use roads, but I really don't want to ride them on the limestone paths, or busy roads.
I'm building a more modern gravel bike for the limestone trails. I'm only going to use 35mm tires, as I will not be riding on rough surfaces. I've got no interest in singletrack, so I don't own a mountain bike.