I will first state that I have been in this hobby for about 2.5 years now. I have MUCH to learn, but I don't quite understand the reasoning of the answer given by rajul.
Aerodynamically, it is supposed to expand the "flight envelope" i.e. the minimum controllable airspeed is lowered, and also to reduce flutter.
I built a 4*120 last winter and the ailerons on it looked like chunks of balsa. (no taper from front to rear). I did read an article, and have been involved in a thread where the flutter-no flutter discussion was discussed. The opinion was divided, as usual. That discussion was mainly directed to the practice of ROUNDING the TE of the aileron. This is my $.02 worth. If the aileron is thicker than the trailing edge, it is not aerodynamicly "slick". Secondly, it looks tacky as h***. I have never seen a full scale plane like that, and don't remember ever seeing models with a "bump" at the wing and aileron. I DID seem to agree with the thread and ideas.... "ROUNDING" the TE of the aileron can promote flutter. I used a razor plane on my ailerons, and shaped them to "flow" with the shape of the wing's TE. I tapered the aileron, from front to rear, to look decent. (instead of a flat piece of balsa). I did NOT round the aileron's TE, but left it square. The 4*120's plans also have the rudder and elevator flat too. I also taperd those too....
nothing drastic, but it looks AND flies great.
PS. As rajul stated... sealing the gaps is a good thing, and sure can't hurt. ...lownslo