I don't get "growling". Never made sense to me as a "style" of singing... so weird that there are people who like it... 
It is just a different way of manifesting and communicating feelings wrapped up in music. Pretty much all singing styles are aimed at that. Clean vocals, for example, are utilized to transmit a calm or maybe confident mood. Growls and brutal vocals in metal, on the other hand, are a direct means of reflecting an ireful and frustrated state of mind.
