I almost want to excuse myself for this topic, I think that it must have been discussed on the old forum at some time for sure...
Anyway, after all these years in the martial arts and some (:-)) hours on message boards I still fail to see a solid functional reason for keeping the back heel planted on the floor in rear hand strikes (gyuaku-zuki, rear cross etc.).
Most reasons were of an apologetic character such as "is good for stability". Since a fight is movement and any transfer of body mass for the rear hand to have any power must move the body forward, this concept of "stability" is not transferable to combat.
A more refined variation of this is the kickboxer who stays on his toes and has no power in his punches and will surely loose balance/fall to the ground in a self-defense situation.
Another "good one" is that it appeals to aesthetics. Could be, but then, art is subjective.
The only somewhat useful explanation (because it's mine :-)) is purely from a "health" point of view:
most people do not activate their glutes enough when walking (due to our sitting lifestyle). This is then coupled/ leads to tight calves and lack of ankle mobility.
(Btw, nowadays I think that kata movement in general if done not too fast and not crazy-combative like may lead to a more graceful and relaxed movement style in everyday life - just an idea)
To sum this up:
Do you have any good explanations for keeping the heel on the ground - be it for fighting or health purposes?