When you adjust the fuel mixture with the needle valve, you are effectively fine tuning the ignition advance. With spark ignition, the spark fires at a very specific point, usually around 25 to 30 degrees before the piston reaches the top. With glow ignition, the exact point of fuel ignition is determined by the compression ratio, hotness of the glow plug, and the richness of the fuel air mixture. A lean mixture fires sooner than a rich mixture. That's why there is such a drastic increase in rpm when the mixture is leaned just a few clicks, not only is the mixture being leaned, but the ignition timing is being advanced also.
The fixed spark timing of our cars and motorcycles is one of the main reasons we don't have to carefully readjust the needle in the carb every time the weather changes.