What does overclocking the memory clock on a gpu do?

I know the core clock and shaders is what is more important. I started overclocking the memory clock (GTX560Ti) but haven't seen any difference in fps or temps at all.|||its the speed that the memory is working at, higher speed = loading textures faster but if its already loaded them all it wont make a difference, normaly memory clock is rather useless to overclock cuz its already fast enough, its what gives the least performace boost|||it just speeds up the memory on load and off load, but it doesnt affect much at all, HOWEVER if you over clock the CLOCK SPEED, then the GPU will work faster and will have a difference in game performance :)