The different durations are meant to target different energy systems. The longer intervals are designed to work on your threshold power, whereas shorter durations are meant to work on your anaerobic capacity, or vo2 max capacity.

But, you need to key the intensity to the duration. 6 minute intervals at threshold aren't very useful. It's when you stretch it out to 15-30 minutes that you start to force adaptations. At 6 minutes, you want to go harder than threshold.

So to answer your question, different lengths of intervals for different energy systems. Which one you want to train depends on where you are in terms of fitness, as well as your short and long term goals.