From my understanding, both scarf joints and volutes are used to increase the strength of an angled headstock. But out of curiousity, why does Warmoth offer both? Isn't a scarf joint or volute alone sufficient enough?
Each feature strengthens the neck in different but complimentary ways. The scarf joint orients the grain better and prevents the runout present in the headstock of one-piece tiltbacks. The volute puts more wood where the neck is otherwise weakest - under the recess for the truss rod nut.
You know the old saying, "Less is more?" Well, when it comes to preventing broken headstocks, Warmoth says "More is more."