The biggest draw to gravel riding for me is getting away from cars/traffic and into areas that are more rural, quiet and natural. I like mountain biking for those reasons too, but dislike being contained to a handful of specific (and often busy) trails that I have to drive to access.
For me, gravel riding is basically just riding wherever I want, regardless of surface. In most cases, my preferred routes include a combination of paved roads, paved trails, unpaved roads, and unpaved trails.