By doing this it allows directors to set up/choose camera positioning and shot lists. This is used in many films and music videos. For example this is an animatic for the Mansion attack in 'Iron Man 3'.
In music videos it is used quite often and for the full track and not just for clips from it, to research for my animatic I have looked at lots of different animatics, whether they're by professionals or amateurs to get a feeling for what I will have to do for mine and whether there are any codes or conventions used in them that I will have to follow for my animatic.
Some good examples for music video animatics come from the band 'The Gorillaz' :
'Feeling Good Inc' Animatic above and 'Dirty Harry' animatic below.
Compared to the final video the two are quite similar and the final video ends up almost identicle to the animatic.
'Feeling Good Inc' official video above and 'Dirty Harry' official video below.
What I've learnt by watching these and other animatics is that mine doesn't need to be perfect, as long as it gets my point across, and shows what I want for my music video. Also when filming I can use my animatic for guidance, but I can change bits and don't have to do it all the same as my animatic.