The SSS (short for "Secret Strat Sauce") was designed for maximizing the tone and character of single coil pickups. It is a medium gain pedal that stays incredibly tight in lower registers, especially with the neck pickup.
The goal was to emphasize the unique qualities of single coils, especially as used by players, such as ones named David and Richie.
The Extra switch is an added bonus that gives a slight low-mid boost. This is especially helpful when presented with over bright or harsh backline amps, or when a bit more muscle is desired.
By no means is it limited to single coil use! Humbuckers sound great as well!