We have kneecaps (the patellae) mainly to make the knee stronger and more efficient at moving and supporting body weight. They act like small, built‑in pulleys that boost the power of the thigh muscles and also help shield the front of the knee joint.

What a kneecap actually does

  • The kneecap sits inside the tendon of the quadriceps muscle at the front of the thigh.
  • When the quadriceps contract to straighten the leg, the patella holds the tendon a bit farther away from the center of the joint, increasing leverage so the muscle can extend the knee with less effort.

Why evolution “bothered” with kneecaps

  • Standing, walking, running and jumping on two legs put big forces through the knees, so extra mechanical advantage is valuable for getting up from a squat, climbing, or landing.
  • The patella also helps distribute force across the knee joint and protects the front of the joint and the tendon from friction and impact when kneeling or bumping the knee.

Why arms don’t have “armcaps”

  • The elbow doesn’t usually support full body weight the way the knee does, so it never needed the same kind of floating bone in a tendon for extra torque.
  • The back part of the elbow (the olecranon of the ulna) already provides some bony protection and leverage there, so a patella‑style bone is less useful in the arm.

Quick forum‑style take

In simple terms, you have kneecaps so your legs can hit way above their weight class: more power to stand, jump, and climb, plus a bit of armor over a very busy hinge joint.

TL;DR: Kneecaps boost the power of your thigh muscles by improving leverage at the knee and help protect the joint, which is crucial for a two‑legged, walking‑running, stair‑climbing life.

Information gathered from public forums or data available on the internet and portrayed here.