Imaging: the illusion of 3D perspective.
The back wall that disappears.
Simple, really.
Only ESL’s and planars in a big room can do that well.
LS3/5a style mini monitors can do it but the scale is smaller.
I guess you haven't experienced it (we can not have heard everything in HiFi world), but omnidirectionals excel in that area. Also, the later Stig Carlsson (OA 5x series) did (does, you can buy a new set called OA 61).
My explanation is, when listening to speakers, the ear/brain need to be able to differ between the direct arriving sound and the somewhat later arriving reflections of the walls/floor/ceiling, the latter preferably arriving from several directions with no discernable phase relationship. Meaning it should be diffuse. There is more than one way to reach this!
PS. To me, 1D is a straight line between the speakers (or depth in a mono recording), 2D is depth, 3D is height.