I seriously cannot believe that people are being insulted here for asking about the dagger.
First of all, daggers were weapons as any other, they were used in combat and the art of dagger combat was taught and practiced. On Scottish Highlands, for example, most people were very poor and could not afford a sword, but almost every adult man had a dagger called “dirk” and knew how to use it, and most quarrels and duels were done by these dirks.
Saying that it’s okay that daggers cannot be used in combat, because “there is no chance” against a sword is nonsense. Sure, no soldiers marched to battle armed just with daggers, but in a life or death situation, any weapon is better than bare hands, daggers were also secondary weapons of last resort for many combatants, for example archers.
So there is nothing wrong on expecting that daggers would be equippable and usable in combat as any other weapons.
The fact that they are not is inconsistent and confusing, I am sure that the devs had their reason, but I would be personally happier if you could fight with daggers too.