Even though we refer to a flat lay in a hammock, that's not entirely true. We still sleep in a slight banana shape. Your torso weighs more than your legs and because of this, your torso drives the legs toward the end of the hammock so that the heaviest part of your body sinks to the lowest point of the hammock (the middle). Raising the foot end moves that lowest point up toward your torso. Now you are stable and won't go sliding to one end.
How many times do we all remember waking up in one corner of our tents because they weren't pitched on perfectly flat ground?