Pilots have been looking for ways to make their airplanes faster and more efficient since there has been airplane ownership. One of the long-targeted spots for aerodynamic clean up has been the gap between the trailing edge of the wing and the ailerons and flaps.
The idea was that reducing the flow of high pressure air from under the wing, through the gap, to the low pressure area above the wing, should increase overall lift and allow the wing to operate at a slightly lower angle of attack, making it faster. It proved to be generally true and gap seals became a stalwart part of virtually all speed mods.