I think partly out of the desire to impress.
If you're skills are at the outer limit it shows that you've gone through hours of hard work. Hard work is respected, doesn't matter what you do, just that you worked hard.
I love to play extremely difficult and complex music, but my respect goes to those who manage to say more with less words. (ie notes). I mean it's good to make music with drama and complexity and difficulty, telling epic stories about heavenly beauty or purest evil, but sometimes i like simplicity, profound beauty expressed with objective clarity and without superfluous excitement.
Like a pure and simple melody, without chords and intervals, just the melody. I think all that we have today started from making melodies without any kind of accompaniment - the point at which simplicity and complexity become intermingled and create worthwhile music.