Single-rods are great for flexible mounting of light accessories such as webcams and light weight LEDs as the can be used with angled M12 threads.
Double-rods provide a greatly improved load capacity and rotational rigidity which is especially useful when side mounting heavier accessories such a microphones. They also feature a lot more mounting options with their cheese plates, offering countless 1/4"-20 threads and their rod clamps also offering M12 threads for more top and bottom rods off the side of a monitor.