If this wasn't covered during your training that's a little depressing.
At idle the engine is still running and developing some power.
Windmilling the airflow keeps the propeller rotating. This means rotating the crankshaft and moving the pistons up and down against the compression stroke.
That takes a lot of energy.
A constant speed prop does better if you pull it all the way back ( coarse) during a glide. You actually feel the airplane accelerating when you do this. Difference is a couple of hundred feet per minute in your rate of descent.
A car with 140hp engine doesn't need 140hp to drive 60mph.
Actually more like 40hp as the car is already in motion and doesn't need to be brought into motion.
Now consider a propeller.
On the ground when you do your preflight it takes quite some effort to turn the propeller even one revolution.
No imagine 1200-1400 rpm windmilling and how much energy that would take.
Technically not 100% correct comparison. Remember the car example? The propeller is already in motion so it does require energy to continue to turn but not as much to get it to turn? Make sense?
Been a while since I've done this but I used to turn off the fuel in C152/C172's as a demonstration.
Engine isn't running but windmilling.
Slow down to just about stallspeed is required to get the propeller to stop.
But the decrease in rate of descent and the increase in glide range is pretty impressive.
Now to get the propeller to turn again you need a lot of energy despite the fact it kept windmilling till about 55-60kts.
You'll need to dive ( in a C172) to about 100-110 kits to get the propeller to turn again before slowing down to best glide speed again.
Now a mechanical failure can lead to engine stoppage as in the propeller stops turning also as fuel or ignition issues keep the engine windmilling.
Especially if you start at higher altitudes it may be a good idea to slow down to get the propeller to stop turning and therefore the engine to increase your glide range.
At low altitude you're better off concentrating on where to put it down.