Purpose of Music?

To aesthetically re-experience our (particularly human) relationship with the world. To quote at length from John Dewey, my favorite philosopher of aesthetics:

"There is in nature, even below the level of life, something more than mere flux and change. Form is arrived at whenever a stable, even though moving, equilibrium is reached. Changes interlock and sustain one another. Wherever there is this coherence there is endurance. Order is not imposed from without but is made out of the relations of harmonious interactions that energies bear to one another...

Order cannot but be admirable in a world constantly threatened with disorder — in a world where living creatures can go on living only by taking advantage of whatever order exists about them, incorporating it into themselves. In a world like ours, every living creature that attains sensibility welcomes order with a response of harmonious feeling whenever it finds a congruous order about it.

For only when an organism shares in the ordered relations of its environment does it secure the stability essential to living. And when the participation comes after a phase of disruption and conflict, it bears within itself the germs of a consummation akin to the esthetic.

The rhythm of loss of integration with environment and recovery of union not only persists in man but becomes conscious with him; its conditions are material out of which he forms purposes. Emotion is the conscious sign of a break, actual or impending. The discord is the occasion that induces reflection. Desire for restoration of the union converts mere emotion into interest in objects as conditions of realization of harmony."

Art as Experience, 1934

Music is everything...we cant live without it. 

It does have different purposes, it depends how we use it. 

In scientific terms, put simply:

Music is a extension of the brain's area of communication put into rhythm and melody.

It's another way we speak to one another rather then tones that associate objects and meanings.

Music is the defining factor of communication through other means then language.

Have you ever heard of music being used in a negative sense?

No, you haven't.

Sure. Death metal, Killing tribal chants and others mean something negative. But it's for something positive in the end.

Music doesn't bring a negative impact to anything.

If you show rhythm to monkeys, they join you. Like another monkey, this can be used to become one with them.

All creatures use this "music" as a form of communication.

It's waves in the air.

purpose?

sorry, I don't think I need such thing

Music essentially is a byproduct of our advanced speech processing. As such, it can be expected to be fairly old. Research on the acoustics of paleolithic caves seems to indicate that chants were in use back then - they still are quite important in religions today. Music with some sort of social function in general can be expected to have been important (rituals, work songs, whatever - maybe even love songs, as classic as the topic seems to be). You also commonly find folk songs that tell stories, so in an age with no or very limited written history, they certainly were a somewhat efficient way of passing on knowledge of all kinds (musical storyteller traditions can still be observed in some African countries, for example; also see Cantastoria).

I am unsure whether there was a lot of "l'art pour l'art" music until monarchs got into sponsoring it. Even then, it tended to have a fairly clear purpose most of the time - many of the early classical composers made their money writing church music. In the course of the industrial revolution and related changes in society, audiences broadened. Finally, selling prerecorded material became possible and proved a goldmine for decades. This whole forum owes its existence to the resulting industry.

Music, to me, is a supplement to my thought patterns. Music induces some crazy feelings in me, some irrational and bizarre, and some that would be expected in a human being. I am so impacted by music, that if something I don't like is playing, I will do everything in my power to move away from it. I become intensely connected to lyrics, and thus that is one of the reasons I don't listen to mainstream pop and rap; most of the lyrics are mindless (something I will never be) and intended just for mere amusement and entertainment. However, of something is tastefully done, I will be interested in it no matter what genre. 

Richard Ashcroft in the Verve's track "Bitter Sweet Symphony" pretty much sums it up for me:

"I need to hear some sounds that recognize the pain in me yeah..."

I sometimes prefer music without lyrics...