"Presuming a weapon, spell, or other source of injury inflicts a variable amount of damage, you have to determine what the actual result is based on the parameters of that variance. Or in other words, if I clocked someone with my sword, in addition to the reliable and steady damage inflicted by my strength and the weapon's enchantment, a roll would be necessary to see what the sword itself did. The general idea is that such variance represents arbitrary factors outside the attacker's control."