Raising the knee up high before executing a kick has obvious advantages in competition as if you throw the majority of your kicks from the same position your opponent will have no idea if it's going to be a high/mid/or low kick, decreasing the chances of them covering up, meaning increasing the chances of you landing the kick and scoring a point.
When it comes to self protection however raising the knee to waist height before kicking is telegraphing what you are doing, so it makes much more sense (to me) to keep it as low as possible to disguise your intentions. This is a good example of what I mean:-
I am of the opinion therefore that raising the knee to waist height is a relatively new idea that has come about once competitions/sport was introduced, and that before that knees would have been kept low. Or do you think there is a valid reason in a self protection context of raising the knee to waist height?