If you mean a position as a back up dancer then the harsh reality is its extremely hard to get the position and sadly those who do it without pay and do "favors" (if you get my drift) normally get...
Actually it is the music that is made before the music video so your real question should be why does the music video have to relate to the music. The answer to your question though is because if it...
it is a video that 45% of the time shows why the singer or writer of the song wrote it, 10% of the time it's just a jumbled up mess trying to start a trend, and the other 45% is just the singer...