Let's go way back, to the 1920's when moving photography was at its birth- around the 1920's we had black and white films and they hadn't quite figured out how to make full voice tracks yet and so almost every film at the time had a music track to it- hence you could say, marking the birth of the music video. This means that music videos are almost 100 years old. The first promotional videos began to appear in the 70's as a means of getting the band more screen time and of course the song itself. With these new and revolutionary videos we began to see dedicated channels appear (i.e MTV) or dedicated shows like Top of the Pops.
As the format of the music video has expanded; so have the means of distribution- at first with the video cassette- an idea used in the 90's often released as part of a collection of promotional materials from the band. And with more and more channels becoming avalible to the viewer we began to see channels dedicated to a specific genre.
Following the advent of the 2000's and the rise of faster internet speeds we could now have video sharing at good quality over the internet, something that has become more apparent since 2005 with the launch of youtube, which hosts millions upon millions of the music videos- giving the bands a much wider audience and quite frankly the viewer can choose what they want with video playlist. Another thing this has given rise to is the whole aspect of User Generated Content allowing the user to download the video (maybe not entirely legally) and make their own edit of it- or creating a new video using whatever footage they want to from games to live footage of the band. This allows the music to be heard by people who maybe wouldn't normally listen to the genre.