Updated: Feb 22, 2020
I started whitewater kayaking in 1991, and at that time paddles had a 90 degree "set", otherwise known as a feather. No one asked any questions.
I started sea kayaking not long after and soon came to the instructor realm. I found that instructors for sea kayaking were teaching that the blades were off-set, or feathered, in order to "cut through the wind", or in other words the idea was if a paddler was going in to a head-wind, a feather to the blade helped.
So I wondered why whitewater paddles also had a set.
Eventually I figured it out. A blade is feathered for the simple reason that you need to lift the paddle shaft angle in order to place the other blade in the water. Without a feather to the blades, you need to contort your wrists to orient the blade correctly.
Depending upon the angle with which you hold the paddle shaft to the water surface, you will need to off-set one of the blades by about that much. A 45 degree shaft angle corresponds to about a 45 degree angle set, or feather. Racers paddle with a more vertical shaft to bring the blade closer to the keel line of the boat, so their blades are set closer to 90 degrees.