Fig 1: Front side of Os Femoris (the femur bone)
Fig 2: Back side of Os Femoris
Fig 3: Underside of Patella, where it moves against the Os Femoris
The femur is, by most measures, the strongest bone in the tetrapod body. Its articulation with the acetebelum of the pelvis forms the freely-moving synovial hip joint, and its articulation with the tibia and patella at its distal end forms the knee joint. These joints accommodate walking, running, and jumping, which are critical activities for the survival of most tetrapods.
At the bottom of the front-facing femur (Fig 1), you can see an articular depression between the two condyles, called the patellar surface. This is where the patella rests. The deeper notch in the back of the femur (Fig 2) provides an articular surface for the many ligaments of the knee joint.
The patella's primary purpose is to provide protection to the crucial structures in the knee. The synovial joint is strong, but if it got damaged when we were still living off the land, it could mean death from an inability to farm or hunt.
Cheselden’s Plates of the Human Bones. William Cheselden, 1814 reprint.