[0] Physical Strain on Rider [Kiteboarding] The traction force of the kite is partly transferred to the rider via the harness loop attached to the harness hook. When "hooked in" the rider uses muscle strength to steer the kite and control the kite power by pushing the bar in and out. When "unhooked" the rider steers the kite using their arms with no depower, which can be very strenuous. [Windsurfing] 
Windsurfing without a harness requires a lot of physical effort, especially in strong wind. With a harness, recovering from a fall or when maneuvering (jibing, tacking) the rider needs to detach the harness completely from the sail, which means that both traction and steering forces are to be countered solely by the rider's muscle. Hooking into the harness again requires additional muscle effort. For jibing maneuvers, muscle effort diminishes as the rider becomes more skilled in maintaining board speed in the jibe.