Does fartlek runs have any advantage over normal runs (constant pace) ?

Yes: Fartlek training is a very effective form of running that, when done properly, boosts strength, endurance and performance. Literally it means "speed play". It incorporates fast, medium, and slow running over a variety of distances depending on the terrain, how you feel, and what you are training for. This variable speed training is far more effective at boosting performance than continuous running.