At least when they got started and for a good while after that, it's pure marketing. The biking crowd that buys bikes is somewhat finite in size. Gravel (or insert other current niche types..gravel probably isn't a niche any longer) bikes were the answer to the eternal marketeer's question, "How can we sell more of the same/similar thing to the same people?" ..create a need, fill it, market the heck out of it and ..boom..the gravel bike industry is born(or insert some other niche type).
On a more practical note, gravel bikes are a great "do everything" bike..like bikes sort of used to be maybe 6+ decades ago. Beyond new models, gravel bikes are also accessible from older off-road rigid mtn bikes or hybrids.. find a light-stiff older frame, build it up as a DB conversion, mount nice, fatter, cushy, fast gravel-street tires and have at it. (naturally, a successful conversion takes some careful planning)
On the newer front, this recent thread comes to mind:
https://www.bikeforums.net/road-cycl...ance-bike.html