My understanding is it is a depth map, which is a 2D layer, not a fully 3D map; think of it as a 3D map from a single point of view rendered in 2D using a predefined gradient encoding.

Not one with 360 view if that's what you're thinking. See it as one of those hard halloween masks, from the front they look like the shape of a person's face, but as soon as you look at it from the side/back you just see a thin piece of plastic that has a hollowed out shape.

They're useful as guidance for other image generations to also keep the same structure, or use it to generate a normal map and have dynamic lighting.