Another factor not mentioned by any of the posters is that instruments
tend to come in families, or choirs, and often are tuned usually a fourth
or fifth apart, in order for them to play reasonably closely spaced chords.
Think for a moment of the incredibly rich sound of a top-to-bottom
orchestral brass chord with trumpets blazing on top and tubas way down
below.  Or the same with the families of strings, saxophones, clarinets,
etc.

Scott Morrison