Even the best stereo speakers have an inherent weakness: comb filtering between the channels. You can easily test this yourself by generating some mono pink no

Even the best stereo speakers have an inherent weakness: comb filtering between the channels. You can easily test this yourself by generating some mono pink noise in Audacity (or your favorite audio editor) and playing it on stereo speakers. Move your head and you will hear changes in the sound like a flanger effect. This is most obvious if you are close to the speakers and in an acoustically dry environment. Compare with the same sound hard panned to a single speaker. This is one reason why a real physical center channel improves clarity for movie dialogue.