Why would pt use 2lb ankle weight for strengthening leg of a senior recovering from fracture? Hurts knee?

Ankle weights. My answer is that i wouldn't do any exercise that hurts. Pain is a sign that your body doesn't like this. I would reduce the weight (to 1 pound, or even 1/2 a pound) till i found a level of weight which allowed me to do the exercise without pain. Do that for a while, then slowly increase the weight.
Help strength. Probably trying to build up the leg muscles after the fracture has healed.