Why do some collets have serrations?

Heavy axial or radial forces, intermittent cuts and off-center applications may cause the workpiece to push back into the collet, spin, slip or walk out of the collet. Serrations located in the gripping area of the collet will add additional gripping power using the same draw bar pressure. Serrations also provide maximum contact area when gripping irregular surfaces such as hot-rolled and cast bars, or cast or forged parts.