Most karateka would agree that extending the arm is the most important part of a punch. At the end of the motion there must be kime, complete focus. Without kime, the punch will not be effective.
But when a punch combines speed and kime, and then finishes by pulling back the punching hand, the punch becomes a focused strike, not simply a push at a target.
As you can see in the following video by Jeff Christian his pull-back motion creates a powerful whiplash-effect on the Makiwara. Although he hits the board in a relaxed way, he transmits the energy like a shock wave.
What about you? Do you snap back your fist during Makiwara training? Do you extend the punch and stick the target? Or do you practice both?