With a two stroke, if you turn off the fuel tap and leave fuel in the carburettor, over time the petrol will evaporate and just leave the two stroke oil. This oil will often end up blocking the carburetor jet. When you turn the fuel on to start the mower, it will take quite a while to dissolve the oil that is blocking the jet. Hence hard starting is more likely if you do not empty the carburetor. FWIW, Ryobi's whipper snipper operating manual says always empty the fuel tank, then restart the engine and run it until it stops from lack of fuel.
