They are used in videos to appeal to men. It does not matter if the song is bad but if you have a good video including a lot of girls then the sing will do well in the charts.
The images used are not just used in hip hip and rap videos but re now started to be used in pop videos. This is as they are trying to appeal to a wider range o people.
The images are also being included in films, television programmed and advertising to try and make people buy the product. It is almost like hypnotising you to do something.
However in music videos these are the only type of women you see.