What is the purpose of a cat's heel (halfway up the leg)?
From @Krupip's comment in a related post:
It's a pivot point where muscles attach to.
Muscles contract, causing the bone to act like a lever, pushing the whole foot down, allowing more force to be applied to the ground.
[The particular leg in the diagram] represents the back leg of a dog/cat, the heel is extended on the blackleg to give the animal more power when jumping.
This is good for pouncing predators, but also the only practical way large animals can jump with any distance, such as horses, which only have the pivot on the back leg and have pillar legs on the front (the bones are inline, there is no modified heel pivot location).
Feel free to edit or replace this answer.