It's all about being smooth through the air.
A wheel with spokes that is turning around will constantly cut the wind with it's spokes and so it will cause a lot of turbulence.
Turbulence means that you have lost energy by moving the air and is therefore directly related to drag.
A disc wheel does not have spokes and so it does not cut in such a weird way, making it to cause a lot less turbulence ... in that way a disc wheel is faster than any other wheel.
A high profile rim is the middle ground between a full disc and a normal wheel ... having better aero but not optimal.