Err, they're mostly used on leaves. At least in my state.

I think it varies and you probably shouldn't have that much confidence stating something you don't actually know as fact. In my personal experience I see more commercial operations using them to blow cut grass. Maybe a higher ratio are privately owned by individuals though. Who actually knows?

In my area they're occasionally used to blow leaves in the fall. Mostly they blow grass clippings off the street and sidewalk. Leaves in the fall are mostly mulched by mowers. It's about 20 times faster than blowing them around and collecting them.