The kneecap (patella) has a very important job to do. It helps the knee joint move and work normally. The quadriceps muscle, the large muscle on the front of the thigh, comes down, crosses the patella and attaches to the lower leg bone. The patella acts as a pulley system to help you use the quadriceps muscle to straighten your knee. This is called the extensor mechanism.
Taking the patella out changes how the extensor mechanism works. Change in the forces around the knee from extensor lag can cause an unstable joint. There's also a loss of protection of the bones in the knee joint.
Lastly, the appearance of your leg will change. This may not matter to you when you are in so much pain. Once the patella is gone you can't get it back so it's a fairly important decision.
Some doctors prefer patients try a conservative approach. This means pain management while the bone heals. Taking the patella out is more of a final treatment option.