Snes9x.com

Its used to reduce that lag from controller to the game, like this: normal - you tap the button, 1,2,3... frames passes then your character in the game responds. reduced input lag - you tap the button, ...4, the emulator subtracts the number you configured, that number in frames. so, you'll loose fr...

Pretty less than expected.
street fighter - yes
super mario - can use, but ...nah
super mario kart - must use, good speed boost after curves

barely remember other games, but often used to restart the game, or to do cheats in game.