Black Diamond Knifeblades are the oldest pitons used for thin cracks. They used to be the only way to get up skinny cracks. Then came Birdbeaks, multiple sizes of the Black Diamond Pecker and the Moses Tomahawk. Knifeblades have fewer uses today but are still an essential part of a big wall nailing rack. The bigger sizes of Knifeblades are called Black Diamond Bugaboo.
If you are a serious aid climber, you will need a few of thesebut just a few. In general, it's better to invest in the Moses Tomahawk and Black Diamond Pecker. Those are not only more secure in most nailing situations, they can also sometimes be hand placed for clean aid moves (you can't hand place a Knifeblade very securely).