There has to be some amount of time spent on finning technique because without it new divers could experience injury, fatigue quicker and just be frustrated. And then mentioning the "why" factor, like impacting the environment and visibility, is stuff you should know from the beginning. You could probably dive without the knowledge, but it's going to be that much easier and more enjoyable of an experience for them if that time is taken right from the start. Just like how to put the fins on and take them off! I'm sure most people could accomplish it in some fashion on their own, but a few minutes of the instructor explaining how and why will just add so much more to the ease and enjoyment of their diving.