Celebrities, including rappers, have been inking endorsements deals with companies since the dawn of advertising. Hip hop music is more popular and marketable to the mainstream now than ever before, so these kinds of deals are on the rise.
It would seem like a no brainer, rap and advertising belong together, but the ad industry didn't always recognize what hip-hop could really do for marketing. This all changed in the 1980s. Each side began realizing what the other could for them.