Not only ramps but also edges for leveling designation, dumbing designation and mining designation
Its so you can have clean edges instead of just ramps that go straight down on the side (had to take the picture with my phone since photo mode doesnt show it)
I suspect they'd have to implement a new "corner" type of mining square to implement this wish, which would probably be hard for limited gameplay benefit. I agree with you—I'd love to be able to make my dumping pyramids cleaner! But as it is I think the only way we can get that is to build it 1 level too big, then dig down the top layer: the corners ought to be clean after that, though I haven't tried it yet. Same in reverse for pits, I suspect.