By treating particles that are close together as a a single mass, this means that less computations on the gravity need to be done. This is ok to do only when the mass is far away from the body, because then the differences in the affect of the gravity for each individual particle will be less.
By treating particles that are close together as a a single mass, this means that less computations on the gravity need to be done. This is ok to do only when the mass is far away from the body, because then the differences in the affect of the gravity for each individual particle will be less.