In the beginning of the 1970s TV was used to promote songs, visually. With the creation of 'Top of the Pops' and its broadcast on TV, artists found that they could sing and be seen by their fans and have a live performance, and if they were unavailable because they were on tour then they would simply play a music video of the artist, so either way the band/artist was being promoted.
in the 1980's MTV sprung up from nowhere and the difference between TOTP and MTV was that MTV was broadcast all over the world, for maximum publicity, and it gave a much broader scope; any genre at the time would have been played on MTV whereas the British tended to lean towards mainstream pop. Also with MTV you could make more music videos to more songs and have an even greater awareness of the artist if they had two or three videos out at once.
The 1990's - 2000's really was the dawn of a new technology. The internet played more of a dominate role in the overseeing of fame with artists through downloads etc. This shunted out of the way a lot of TV viewed music videos. The internet also allowed artists that weren't as famous to upload videos onto sites at any time of the day or night, and it would be broadcast all over the world.