Think of all the sounds in a mix being contained in a triangle with one point facing the listener.
I usually draw it as an upside-down triangle, with the listener at the bottom.
The louder (closer to the listener) a sound can be, the fewer sounds can fit alongside it. The quieter (further away from the listener) a sound can be, the more sounds can fit alongside it. If all sounds must be equally loud, then they all end up far from the listener.
If one sound is close to the listener and all the other sounds are in the background, the mix can seem stark. If sounds can be spread around the triangle, with a few sounds close by and most sounds further back, the listener will experience an engaging and deep sound stage.