Comment by mrob

It's a clarity problem too. Stereo speakers always have comb filtering because of the different path lengths from each ear to the two speakers. It's mitigated somewhat by room reflections (ideally diffuse reflections), but the only way to avoid it entirely is by using headphones.

Try listening to some mono pink noise on a stereo loudspeaker setup, first hard-panned to a single speaker, and then centered. The effect is especially obvious when you move your head.