If you want to generate a convincing three dimensional sonic image, you need all the pieces in place. The more you add, the more convincing the illusion.
For example, a simple two-way stereo setup can present a limited but convincing image. Add a subwoofer to the mix and suddenly you've gone from two dimensions to three.
What's fascinating about this is the fact that the subwoofer is only filling in the smallest of blanks—the frequency region of perhaps 60Hz down. And yet, the 3D image seems to extend all the way up and down the frequency chain.
I think of it like a jigsaw puzzle. You can't yet see the picture it is trying to form until enough pieces are in place.
Fill in the blank areas and suddenly it all becomes clear!