So, just to satisfy my curiosity, what would constitute a necessity? Convenience?
ASME IX, QW 301.2: "The performance qualification test shall be welded in accordance with qualified Welding Procedure Specifications (WPS), or Standard Welding Procedure Specifications (SWPS) listed in Mandatory Appendix E, except that when performance qualification is done in accordance with a WPS or SWPS that requires a preheat or postweld heat treatment, these may be omitted".