they're used like rockets or hand grenades, the have a kill radius as opposed to the standard m203 shell that has to have a bb hit you. when you throw a grenade, if you're within 10-15ft of it (depending on the field) when it explodes you are considered dead whether you are hit by one of it's bbs or not. that same goes for the footballs, they're especially useful when clearing a room because unless it's a large room if you fire your rocket in there everyone in that room is instantly dead.
does that answer your question?