Using a block to defend a punch is only the most basic, elementary application of the move as the opponent is still free to launch a second and third attack, maintaining the initiative. A block that only blocks is a lost opportunity. The block should be seen as an offensive move to simultaneously accomplish one or more of the following:
1. Hurt the opponent's attacking limb causing physical and/or mental disruption to momentarily freeze him.
2. Move the opponent out of position for additional attack while putting you in position for a counter, usually aided by some tai sabaki.
3. Set up a grab with the blocking hand to gain control of the attacking limb.
4. Allow the blocking motion to extend or redirect into a strike with the same hand in a continuous motion.
In all these cases the "block" is used as a preliminary move in an offensive sequence and gains you the initiative. In essence, there is no defense - only offensive moves that may serve a defensive function as you close to the target. One must be intent on striking the opponent (to paraphrase Musashi) and all things should be executed towards this end.
IMO, realization of this concept and basic demonstration of it is a good criterion to judge a beginning black belt. Being able to consistently execute it is a mark of a mid-level black belt. I think this is the main, fundamental concept of karate combat and most all training after the first couple of years should be geared to its eventual achievement.