I have a hard time understanding that too. Once you hit level 20 you can use whatever weapon you want. But let's say two people are using a weapon that does 300 damage. One person is level 25 and the other person is level 30. The person who is level 30 will be doing a lot more damage to enemies even though they are using the same weapon upgraded the same amount. So if the weapons do the same amount what stat increases when you level up that makes you do even more damage?