Certainly there are guest stars and collaborations on albums of all sorts (the new Brad Paisley features duets with Carrie Underwood, Blake Shelton and Don Henley) but in rap, it’s an expectation.
First you have the apprenticeship phase, where a rapper makes guest appearances on other people’s albums before releasing his own. Eminem made his splash rapping alongside Dr. Dre before he dropped his first solo album. Nicki Minaj had done about a dozen celebrated guest turns before her album came out. It’s an effective way to introduce an artist to the fans and build excitement so that even a debut album can be a smash hit upon release.