As far as I can tell, the human knee is fairly similar to the elbow in terms of range and type of movement. They’re both basic hinges, right?
So why is it that our arm bones just meet all nice and simply at the elbow, while the knee has some random little bit of bone floating around? What purpose does the patella serve?
Your kneecaps keep your knees from bending in the wrong direction.
They also protect the joint from impact damage. Legs are in a pretty hostile environment.
But if that’s the case, why don’t we also have elbow caps? Is it just that the knees bear more of a load?
Well, I’m not an anatomy expert (my first answer was simply repeating what I had been told in biology class many years ago.)
I would suspect that the fact that the elbow has two bones (ulna and radius) connecting to one bone (humerus) makes the elbow joint a bit more complex than the knee joint. Your “elbow cap” is built-in to the ulna, as you can see on this page.
The knee and elbow joints are not all that similar anatomically. See images here(elbow) and here(knee).
The kneecap, or patella, is actually what is known as a sessamoid bone. It forms from an ossification center in the patellar tendon. The term ‘tendon’ is typically used to describe a connective tissue band connecting muscle to bone, but the patellar tendon connects bone to bone. This is because developmentally, the patellar tendon is the band of tissue connecting the quadriceps femoris muscle group to the tibial tuberosity. The patella itself forms later in life.
When comparing the elbow and knee joints anatomically, you can see from the above pictures that the olecranon process of the ulna serves to make the ‘hinge’ part of the elbow joint and provide some posterior protection to the joint. The knee has no process for hinge stabilization and relies on a complex group of ligaments (ACL, PCL, MCL, LCL, etc) to provide stability. The patella forms to strengthen the extensor apparatus of the knee and to provide some protection to the joint.
hehe, Phase42 uses Google Images too.
That’s the beauty of having arthrogryposis: I don’t have to deal with trivial body parts such as knee caps.
Of course my knees don’t bend so well, but you take the good with the bad I suppose. 
Unlike the elbow, a knee basically supports half the bodyweight (and the patellofemoral joint reaction force can be eight times bodywight doing deep knee bends, or three times bodyweight when climbing stairs). It thus stands to reason that the knee must be a more durable structure than the elbow.
The patella increases the distance (moment) of forces across the knee from the axis of rotation. This increases the mechanical advantage of extensor muscles, as well as increasing the functional lever arm of the quads. The kneecap also changes the direction of pull of the quad mechanism, which strengthens the knee structure by permitting bigger loads.
Most of the big blood vessels to the leg are located behind the kneecap (posterior leg). The kneecap helps protect big blood vessels from injury.
Babies and elderly people often fall. The kneecap helps absorb shock during a fall to prevent other damage. It’s fairly hard to break a kneecap.
My dad fractured his on one of those plastic flaps that keep the cold air inside a cooler. I’ve been hit a few times in the kneecap but no breaks so far. Maybe he’s just getting older.
That’s what I learned in my biomechanics class.
Ditto what Dr_Paprika said. The patella acts sort of like a pulley, to allow the quads to extend the lower leg through a wide range of angles. Without it, it would be almost impossible to extend your leg again once it was flexed all the way.
I use mine to find small objects I dropped on the floor.
A heavy blow to the kneecap can result in fracture. No, not my bookie with a baseball bat, but the lowest edge of the dashboard back when such things were made of metal…this happened in a head-on car wreak.
The kneecap was fractured in such a way as to look like a cut pie, with several wedges all emanating from the center. The cure was to put me into a hip to ankle cast for 2 months…nowadays, they would probably pin it all together.
Also, I believe that the kneecap keeps the patallar tendon anchored in the middle of the knee, rather than slipping to the side.