Even at idle the prop will still produce a little thrust, and you will have the comfort of knowing it is there.
It's a good way of demonstrating the environment you will find yourself in, in the event of a real engine failure. It sounds and feels different than simply pulling the throttle.
One of my instructors routinely pulled the mixture overhead the field at 2,000', and it was left that way until roll-out on the runway. After the initial nervousness, it is a very satisfying feeling to do it.
Just my experience, of course
p.s. I knew 'of' an instructor who would position the aircraft on a high final, give control to the student, pull the mixture then turn the ignition off and throw the keys over his shoulder into the back. Now THAT is daft!!