It raises the center of rotation of the boat higher. With boards in the water resisting the sideways motion of the boat, the boat hinges around a point somewhere under the water where the lateral resistance of the boards resist the side force from the sails. If you raise the boards, you raise that pivot point closer to the surface of the water giving the sails less leverage to heel the boat. When you raise the boards, you trade off a little resistance to sideways travel for a higher center of rotation and less wetted surface. This helps increase your speed potential downwind because of two things: 1) your speed increases so the boards are generating more lateral resistance and 2) a ~little~ side slip downwind isn't hurting you as it is taking you deeper down the course*

*you might think that a ton of side slip downwind would get you to the mark faster but you also have to consider that a hull sliding sideways in the water is MUCH more draggy than a hull traveling in a straight line. If you raise your boards completely out of the water you will get a lot of side slip and speed will be reduced.