A Fartlek run is a run where your speed is going to vary for random distances. Sometimes you run fast, sometimes you run slow. The distance is usually relatively short, maybe a few miles.
You should do Fartlek runs for no other reason than the name. It's fun to say “I'm going out to do a Fartlek!”
If you need more justification, you should do them because they train your body to make the adjustment from running fast to running slow to walking and back. In an ultra, you'll probably make this transition several (if not many) times. This training run will help.
You also get some of the benefits of speedwork without as much “vomit” danger. When you begin to reach that threshold, you just slow down.