Forward facing dropouts are angled to match the angle of the rear brake caliper, so as you move the wheel in the dropouts, the rim remains the same distance from the calipers, such that the brake blocks do not need to be moved in the caliper slots to maintain the proper position relative to the rim's braking surface. Rear facing "track" dropouts are truly horizontal, such that moving the wheel changes the rim's distance from the brake caliper, requiring readjustment of the brake pads. This makes sense, since fixed gear track bikes do not normally have rear or any brakes.