Part of the development of an athlete is to incorporate higher intensity work. Fartleks or speed play runs allow you to introduce bursts of higher intensity into your workouts. These runs are not maximal speed work efforts but runs with varying speeds to drive up overall intensity. The increased intensity challenges you to run with optimal form, and also challenges your aerobic system to develop without the injury risk and recovery requirements of true speed work.
I like to use these kinds of runs during the base season to develop stamina, before the more difficult and challenging workouts begin. Fartlek workouts act as bridging workouts in a training progression.
As you run Fartleks, focus on running form and relaxation as you surge and increase speed. You do not want to increase speed so much that you need to recover and slow down significantly. The ability to sustain effort through and after all fluctuations in speed is what you seek.
Have fun with them.