The human ear, due to it's shape, adds subtle filtering to sounds. Things will sound marginally different if heard from in front or behind you. The size and shape of your head, and the fact it physically blocks one ear from the other, does a similar thing to the left|right field while also inducing extra phase differences as well as volume differences.
You won't be able to consciously notice any of these subtle changes because that is how you normally hear things, but all these things put together mean our brain can calculate in 3D space where sounds have come from, not just in the left|right stereo field our two ears would imply.
The "head" tries to replicate this with mics in its "ears" to record what a person would hear from that point in space. When played back through headphones you should get a true 3D sound field.