height

Every jump in ballet is preceded by a counter-movement, namely, a plie, and this is exceedingly effective in getting more height out of a jump (try jumping from straight legs and see what happens), not to mention protecting the achilles tendon from injury. In fact, people have studied jump height and found that maximum vertical height in a jump is higher using a counter movement than not (3). So it makes sense that we bend our knees before jumping.