Pfffffffffft..plain and simple - communication:
"The Origins of Music
In its most primitive form, music may evoke the sound of the elements - earth, air, fire, water. Since humans have always been imitative animals, it seems natural that nature itself should have provided the scope for the earliest music and the materials for the earliest musical instruments. Where these materials -- sticks, stones, bones, bells, reed pipes or whatever -- were not available, the human voice was a more than adequate substitute, relieving loneliness, making contact with other people, reflecting the rhythms of manual labor or simply of walking, celebrating victories, or paying tribute to primitive deities -- often in combination with dancing."
http://www.thereference.com/music/
I believe strongly that any form of music is made to communicate something - happiness, fear, a picture in the mind's eye, a metaphor, anger, beauty of instrument, etc. Why bother creating music if you have nothing to say? Even in its simplest form - perhaps you like the cello and you want to let everyone know how beautiful the instrument is, and you take the time to write pieces that would most reflect that instrument's beauty. What about guitar solos? The best solos are a work of art and a joy to listen to. You think, "how the HELL does he do that?", and you gain more appreciation for the guitar. Communication in its primitive sense, as is today.