By keeping the internal parts of an engine such as injectors clean, you ensure the best possible spray pattern so the air-fuel mixture in your cylinders is optimal which also means optimal combustion. This in turn means that less fuel is left unburned, which causes less carbon deposits and particals to be left behind in the combustion chamber. You also ensure that your valves won't stick and will close properly so no air-fuel mixture can escape the combustion chamber before it is ignited. Therefor you use less fuel to get the same amount of power output from your engine, thus decreasing the fuel consumption.