Some areas just have a lot of aggregate that merits using tires bigger than 40mm.

Oftentimes people say that if the surface demands tires bigger than 40mm, it's better to use an MTB. I think this misses the point of what an MTB actually is: optimized for technicality. Not all routes with chunky gravel are technical. If you've got roads with chunky aggregate that aren't technical, and especially if your riding also has pavement mixed in, using fat fast-rolling tires on a road-esque gravel bike can be significantly faster and more fun than using a mountain bike.

And in my area, even on roads that are mostly pretty smooth, sometimes there's spots where giant aggregate was dumped to ensure good drainage or whatever on the cheap. Here's a particularly nasty one from a recent ride:



I see wheels and tires damaged all the time on that kind of crap, and nobody wants to hike-a-bike every time they see a rough patch. Bigger tires reduce the risks, protect equipment.