I think it's important to remember that a lot of the rapper and hip hop artists were also starring in the movies that some of their music was being heard. This means two things to me. For one thing, it take advantage of the chance to showcase one of its talents, like most films do that include a singer in the cast. Second, it is another way to express the emotions and experience the film is getting across. A lot of the music heard in the hip hop movies is probably what the characters would be listening to in reality, giving the spectator an even deeper sense of being in their world.