By shifting their center of gravity to the inside, that can keep the bike as upright as possible and maintain maximum grip in the corner.
I've used that technique (on a bicycle) going around corners where traction is suspect. Let your body do all the leaning while you keep the tires as upright as possible. Works good on wet cobbles.
But where traction is good, I agree that leaning increases the contact patch, and will get you around a corner fastest and safest.