In the karate kid film, the sensi has him waxing a car. He complains that it is useless make-work. A bit later on, he gets into a fight. The waz-on-wax-off motion was actually a block to a punch in the face. The constant practice trained muscle memory, and power, allowing him to implement it perfectly.

In short, it’s an apparently useless practice, that trains a useful skill better than direct practice.