its all about the money to reach different markets when a pop star adds a rapper instead of being played on z100 it also can be played on hot97 take for instance mariah carrey new song with cameron its taken from which didnt do that well she took it added vocals now he can profit from it which might make people buy his album on the other hand when ppl are on the same label record execs. might add a rapper youve never heard of to get there name out on the streets;)