Spellcasting weapons are items that are equipped in a weapon slot and cast spells at attackable enemies. The number of casts can optionally be limited by specifying charges. If all casts are used up, the character attempts to attack normally.
A side effect/feature of this is that the normal attack will increase magic skill experience.
Along with the animation override system, weapon spellcasting is used to implement spellcasting gauntlets that are equipped in the melee weapon slot. Although this is not an ideal way to implement such a feature, this is just how it is.