I have recently learned the principles of the standard twelve-tone octave applied to major and minor scales. I understand each scale is defined by its main note, and the pattern of intervals, of half versus full tones, ascending the octave.
The C-major scale, for example, shares with other major scales the interval relationship that the half-tone interval mediates the third and fourth tones, as well as the seventh and eighth (final) tones, with all other intervals being full tones.
Since musical structure is carried essentially by the relationship among tones (the ratio of physical frequencies between any two tones), not by a tone itself (the absolute physical frequency of any single tone), it would seem to follow that any composition based on the tones of the C-major scale may be readily translated to any other major scale with full preservation of the musical structure. Of course, many compositions are described as following a major key other than C. In all, twelve major keys exist, corresponding to the twelve tones into which the octave divides.
In musical terms, artistic or technical, what is the reason for all compositions using a major key not simply using the same particular scale? For example, what is the reason for some compositions being written in C major, while yet others in G major, if both keys share the same essential musical structure?