The benefit of a spiral descent is that you can increase the G loading which helps keep the speed down due to increased drag while maintaining a high descent rate.
Actually, if you increase the G loading, you'll decrease the rate of descent. The object is to REDUCE the vertical component of lift, and a 45 deg bank will reduce it to .707G. That will give a greater descent rate at the same IAS and 1G apparent.
A turning descent can be advantageous near an airport where initial course for return is not important, and also to keep the airplane inside a smaller lateral area.