Fartlek runs are good for challenging yourself in a playful manner. You pick a landmark in the distance like a tree, stone, lamp post or corner, and run to it as fast as you can. After that you return to your jogging speed. Applying this principle during the run you can mix up your jogging/walking pace with high intensity sprints/jogs. These runs are usually for a determined amount of total time like 30 minutes.
These runs are especially good to avoid that you get bored with your training regime. Not necessarily for people who are overweight, but definetly for everyone who can switch between a fast paced walk and a jog.
Usually fartleks are incorporated in advanced running programs to work on speed and bring up average pace for long ditance runners.