I've actually never done them but the two ways they I could see them being effective are
A. the aforementioned CU version where the objective is to do something actually easier than a sustained tempo in your base phase while still getting in solid work,
or
B. doing parts alternating faster than your 60 minutes race pace with parts at that pace or slower which help you change gears in a race setting. These would be more for late season training when you are closer to racing, as they are potentially harder than a steady tempo.
I think both are great workouts but that your early season stuff (think September in an NCAA time schedule) is probably best as true steady paced tempos.
Hey maybe I'm wrong but that's how I would do them. I'm open to the idea that all three types of tempos I have mentioned are good at different times.