The second given meaning of "raw" in Merriam-Webster's probably best suggests this adjective's use in describing music:
"2 a (1) : being in or nearly in the natural state : not processed or purified (raw fibers, raw sewage) (2) : not diluted or blended (raw spirits)
b : unprepared or imperfectly prepared for use
c : not being in polished, finished, or processed form (raw data, a raw draft of a thesis)"
It would seem that "rawness" is perceived a positive quality in much popular music as it implies authenticity. This may be especially true in hip-hop circles where the credo of "keeping it real" is of great importance. For instance when discussing his reasons for wearing a mask, underground hip-hop luminary MF Doom had this to say:
"The mask represents everybody to say that yo, nothing matters, the brand of clothing, none of that matters, it’s about how you spit and how the beats is raw, thats what it’s about."