What I have known over the years is that even if you put the gear in neutral, there still are some parts in the gearbox which keep rolling(spinning).
Let me try to explain, when we try to put in a gear, the gear shaft and the gearbox motor's tensioner engage with each other. If you put it in neutral, they are just disengaged but the gearbox tensioner still keeps spinning. This takes a little load from the engine hence it is recommended to press the clutch fully before cranking the engine as this cuts out the supply to the gearbox from the engine...totally !! Applies to both P & D !!
Example: Ever tried(accidently) to change the gear without pressing the clutch fully while the car is running ?? A big cranky sound will be the result !! This is because the holding joints of the gear shaft and the gearbox tensioner are trying to come out of each other's hold but the gearbox tensioner is still spinning !! Keep doing this and your gearbox will start becoming very loose because of the wear and tear and eventually will go for a six !!
When the clutch is pressed fully, the tensioner stops spinning and hence makes the way for the shaft's arm to come out of the hold.
Hope this makes sense, could not find any pictures to demonstrate !!