Listening to a singer playing a guitar, the singer's voice comes from a higher point then the guitar sound. Now we know, that we locate the places of instrument because of the ability of our hearing system - and it has nothing to do with our eyes. The ability to locate instruments - is a natural ability of the human being and has nothing to do with musical talent!
The goal of two speakers is to give us an IMAGE of a stage with all instruments and vocals exactly like we can hear it in a live concert. The instruments in our IMAGE must be in FOCUS - it means, while listening to music at home with our stereo, we can locate the instruments on the stage at the exact place although the stage is in our mind. The more easy it is to us to locate instruments and vocals , the better focus of image our system gives us. As we know our brain compares the sound of both ears, it's essential that if the same sound comes from two speakers, (let's say a guitar that plays at front center - and therefore must be played at both speakers exactly at the same time and same levels) will be exactly the same sound. If some unwanted differences come from one speaker, our brain doesn't "see" that it's the same sound and therefore the instrument comes out of focus. And because our brain is so fast and so accurate the - system must be as fast and as accurate to "match" our hearing system ability.
When listening to a good stereo system, it is clear from
the start if it creates a good image. If it does, the speakers are transparent
to the listener - he can't feel that the sounds come from speakers - it
appear they come from the "stage" on front, from points between speakers,
above speakers and points left and right to left/right speakers.