When you hit a single mob or player with a spell, or some kind of crude inertia-based impact utensil, the game will work out how much damage that target takes. This is based on the various offensive properties of you and the spell or utensil, as well as the defensive properties of your target. The same is true for area-of-effect (or AoE) abilities, though these tend to do less damage to a single target. Add in some more targets and, while it's still fun to do lots of damage to one of them (with the casting and the poking with sticks), you may have a chance to do damage to all of them at once. Let's say that you can do 2500 damage to a single target with one spell or stab, but can only do 1000 damage to a single target with your AoE ability. If you have five targets that you can hit with your AoE, then that will do a total of 5000 damage. Already we're having more fun than just beating on the one target.