In RPG Maker, weapons can be equipped to actors.
You can also equip them to enemies if you use plugins like Enemy Equips.
Weapons can provide parameter bonuses like extra atk and agi, but you can't specify a damage value for the weapons themselves.
For example, if you wanted to have the normal attack skill's damage value to be equal to the amount of damage dealt by the weapon, you would be unable to say this directly.
You could say that the "atk" bonus from the weapon represents how much damage the weapon can do, but if you were to reference the actor's atk, you would be including any additional atk bonuses that aren't part of the weapon.
This plugin allows you to define weapon damage formulas, which can be used in your skill formulas if necessary.
By separating the weapon damage from your atk, you have more control over how your want your skills to be set up!