The reason is quite simple. You can specialize 2 weapons in 2 different aspects and use whichever is most suitable. One way of specializing is to increase base damage, while the other is to add critical effects like bleeding. If you're attacking an opponent you need to finish off fast, just choose the high attack sword and finish him off. However, if it is a stronger opponent, and you think you need to run around a bit to heal in between fighting him, choose a sword with a critical effect like bleeding.That way he takes damage even as you run around healing yourself. If you are faced with opponents with powerful weapons, it's good to have a sword with disarm capability etc.In general it's a good idea to keep a sword with critical effects, and a sword with high damage at any point of time.As for the best (steel) swords too, the "high damage" one is a 100% increase in damage+ignore armor, and the "critical effect" sword has something like 50% chance on some effects (I don't recall which) . As such, using two kinds of steel swords are not mandatory.But if you master it, you'll definitely have an advantage.Otherwise you can simply focus on using one type of sword, and use the other slot to carry weapons for trade. Every time you get a better weapon you can't just throw the old one away. It's much better to trade it in.