You are right that if the boat was headed into the wind the mainsail would come down without help. My problem in pre autopilot days was that the boat would fall off when I left the wheel to get ready to lower the main. The downhaul helped me do what I needed even as the boat fell off the wind. This was all on a long ago boat. Now, with my trusty autopilot, I don't need to worry about these things anymore.