The panning concept is derived from the film world and specifically the word panorama. This word means to have a complete visual view of a given area in every direction. In the audio world we speak of panning sounds in the stereo field so our audio view analogy would be to hear all the sounds in a performance from their proper location and dimension.
The typical audio mixer has pan controls which can adjust an individual tracks in the right/left stereo field orientation as the mono tracks mix into a stereo mix buss. .A common practice is to place the instruments where they belong or where they would be physically located on a real stage; stage left instruments being panned left and stage right instruments being panned right.This is in keeping with the goal of replicating a real life performance .
Panning sounds of similar frequency distribution and volume to opposite sides of the stereo field will give better definition of the sounds where panning them together will tend to reduce their individuality and clarity. When sounds in the same panning position have the same eq profile, the loudest track will dominate by covering or masking the lower volume track. If a stereo pair of tracks is being mixed, the convention is to hard pan them left and right as they were recorded .
Often within a song there will be tracks that are out of phase with each other and when panned in the same position, track content will be lost or diminished thru phase cancellation. Panning these out of phase tracks apart from each other prevents this from happening and the tracks true character can be preserved.