A few reasons come to mind. First, fartlek is extremely effective for our purposes as a fat loss and general conditioning tool. Interval training has been shown to be the most effective fat burning form of cardio , even more effective than either high or low intensity aerobic training. The exact mechanism of how this works is still unclear, since interval training does not burn as many calories as the other two methods, but it is thought that interval training creates a significant oxygen debt, which signals to the body to preferentially burn fat after the workout. In Tremblay's 1994 study on interval training, it was found that small amounts of interval training were greatly superior in terms of fat loss to much longer periods of low-intensity cardio.
Fartlek is both a great mental and physical challenge.
Originally, as I said, fartlek was developed for runners. However, the principle of fartlek can be applied to a variety of chosen activities. Since with my crunchy knees I don't run unless a person with an axe is chasing me, I have to find other options. One of them is stair climbing. Simple, but a very effective use of 20 minutes.