Probably for the same reason people cave in knees naturally when they're coming out of a squat - seems to be a stronger position.
Whether it's a result of muscle imbalance (adductor vs quad strength) or rather a question of biomechanical advantage - I'm not sure.
One volleyball coach recently told me that that's how they teach ... hm what's that called...when you throw the ball up take a step and hit?
Not sure about this technique and knee health. I don't teach that to anybody and experiment only with myself.