Bipolar hemiarthroplasty - posterior approach - knee immobilizer?

I had surgery for fractured right hip after a fall. Posterior approach. Dr requires use of a knee immobilizer on right leg for 6 weeks post surgery at all times when I am not walking. I want to understand why, but he will not explain. I feel it impedes my recovery, preventing me from bending and flexing the leg, making my leg hang awkwardly from the hip when seated without something to prop my foot on. He claims I am increasing odds of dislocation if I don’t comply. I do wear it at night to prevent myself from curling into fetal position, as that would certainly violate 90 degree rule, but otherwise I just don’t see the point. Can anyone give me a reasonable explanation of how hobbling my (arthritic)knee helps my hip and prevents dislocation?