I am not sure if rapping helps, but i love music, including rap, and i often, especially when i was younger, used to rap alongside my favorite rap songs, including ones in my native language aswell. Sometimes i made my own rap lyrics. I dont want to say it helped me alot, but it is some kind of exercise, and, IF ANYTHING, it helped me control and train my breathing, because you learn when its best to take breath to be able to rap fluently.