Bullets are most effective if they are delivered within their designed terminal velocity range. Also within that range there seems to be a range that imparts the greatest effect on living tissue. Hit a deer with a blazing fast bullet and it is liable to shed its jacket (or petals from a monometal) and whiz right through, seemingly with very little effect on the animal. Too slow and lack of penetration comes into play.
Ballistics helps you deliver the bullets where you want with the amount of desired energy. And yeah, it's fun.
