Purpose of a music promo video

All music videos act as a product of a brand, in the hope that the audience will be inspired by in order to "buy into the brand". For some artists the image is much more important for them than the quality of the product. The Spice Girls are an example of this.

This is because they each have their own nick name that refers to their image. 

Sporty spice is dressed up in sporty clothing.

Posh spice is dressed up posh.

Then you have baby spice who's the youngest of the group or scary spice.

Finally you have Ginger spice because of her hair. Therefore, even though, these girls don't have much talent for singing they all present their own image which gets viewers to buy their albums. 

Other times it's the song and meaning of the song that is sold, and the artists image that has little to do with the sale of the song.