Bone is traditionally preferred for making nuts and saddles on quality instruments. Its hardness and density contributes to good tone, it polishes well, and it allows more precise filing.
Shaped saddles will save you a LOT of work—a head start for the novice, and a timesaver for the busy shop. The sizes have been carefully researched, and accurately radiused, so that minor trimming, slotting, and sanding are usually all you'll need to do.
Blank saddles have oversize length and height, for custom shaping in your shop. They're ready for trimming, filing, sanding and slotting.