I'm no professional, so I'm gonna guess two things: Unloading the inside pedal, as most of the rider's weight should be on the outside pedal, and the angling of the knee is to allow the torso to get lower to the ground, lowering the center of gravity.

Or maybe it's just more comfortable. I've always angled the inside knee toward the turn unconsciously. Though it seems easier to load the outside pedal with the inner leg "kicked out."