Many less well known artists are expanding the rich of their art through file sharing. This is particularly true of musicians. Surveys have shown that most people who engage in file sharing are not, in fact, stealing they are engaging in a kind of try before you buy. Many of these people would not be willing to buy unknown artist if they could not try them out first. File-sharing allow artists to benefit by tapping into that segment of the market that insists on trying first.
File sharing, even on YouTube, is against copyright laws and artists do not benefit financially, at least not directly. Files being shared can increase an artist's notoriety, but not see any financial gain from such exposure until later. File-sharing is a two-edged sword--there may not be an monetary gain, but an artist's popularity could spread--just look at "Gangham Style" from South Korea's Psy.