You can use "junk" weapons to craft special boss weapons using the soul items obtained for defeating them. Boss weapons need an upgraded weapon of a specific type -- greatsword, ultra greatsword, whip, etc -- as well as the boss's soul involved in the recipe. Keeping a weapon of each type is a pretty good idea if you don't know any of the boss weapons' attributes, in case you find that you want to make a particular one. You'll be able to craft these special weapons once you talk to specific NPCs.

You can use "junk" weapons to craft special boss weapons using the soul items obtained for defeating them. Boss weapons need an upgraded weapon of a specific type -- greatsword, ultra greatsword, whip, etc -- as well as the boss's soul involved in the recipe. Keeping a weapon of each type is a pretty good idea if you don't know any of the boss weapons' attributes, in case you find that you want to make a particular one. You'll be able to craft these special weapons once you talk to specific NPCs.

EDIT: Nevermind, in DkS2 you only need the boss soul to make the item in question. I was thinking of DkS1.