Let’s take a look, we have a drummer in the centre of the stage with a singer in front, to the right we have a guitar player and to the left we have a bass Player, stand in the sweet spot (Centre) and you will hear the full band, walk over towards the bass player (On the left) and you will find his sounds starts to dominate (Gets louder) with the guitar player being more difficult to hear clearly, walk over to the right and the guitar player dominates with the bass player being less clear, if you have a mono system (Doesn’t matter the number of speakers) then when you play a recording of the band everything will sound at the same volume no matter where you are in the room, (Not natural) but if you use a stereo sound system (Placed left and right) then as the bass player will be coming mainly out the left speaker and the guitar player mainly out the right speaker, you will get the same effect as if you were listening to the band live, as the volume of the players varies depending on where you are in the room.
Believe it or not stereo was not designed to improve music (This was just a bonus) but to improve film, as when someone talking walked across the screen the voice always came from the centre which was very unnatural, (The ears did not hear what the eye saw) hence stereo came about to solve the problem and make things more realistic. (Of course in the real world we hear sounds from all around us, not just in front of us, which is why we now have multi-channel surround systems to fill in this gap)

Bill