I don't understand the Usage of Stretching in the Exercise, because if I walk, my bodyparts won't get longer ;)
That is a logical assumption, however, speaking from experience, when it comes to animating legs without IK stretch it becomes such a pain to deal with.
Often you just need the leg to go 1 cm further but it can't. That requires you to edit so many other poses in order to adjust things so that foot is where it needs to be.
Having stretch even for realistic animation means the animator is in full control. If it needs to stretch an extra 5 pixels, no worries.
Also many characters have different designs. If you look at the VonnBot Rigs for example. They are robots that have a stretchy limbs. Well only part of the leg stretches (the thigh) and the Shin stays the same length because it is made out of a solid chunk of metal.
They use this same underlying system although only the thigh is setup to stretch.
Hope that gives you some ideas.