Using mirrors (or appropriate mirror/s) is a very important part of getting information for safe driving. A learner will use his or her mirrors out of habit rather than out of a need for information, so it is important to reinforce the reasons behind the use of mirrors.
There are several situations when mirrors should be used: before changing direction, before changing speed, before signalling and just generally keeping track of the vehicles behind.
If a change of direction is needed, then the appropriate door mirror is used to check for any vehicle overtaking you.
Rear View Mirror
On a risk assessment scale, checking the rear view mirror isn’t so important compared to checking the door mirror. But, if a vehicle is closely following you, then an early signal from your brake lights should make it safer to slow down. Plus, if an emergency vehicle is behind with only the flashing blue light on, you might have to re-assess whether you can slow down without inconveniencing them.
If you wish to speed up when the speed limit increases, check your mirror before doing so just in case another vehicle is overtaking you which would put that driver into more danger as they endeavour to get in front of you.
If you observe someone is about to overtake and you want to turn right, to (hopefully) stop them from doing so an earlier than usual signal is needed.
I have given “when and why” reasons for the use of mirrors but not the “how”! The quicker the mirrors are used the sooner the driver can focus ahead again, so moving the head to glance in mirrors should be discouraged (apart from the nearside door mirror) and a quick movement of the eyes should be used.
Mirrors and reversing
“Mirrors are for position and windows are for safety”
There is nothing wrong with using mirrors as an aid to reversing as long as the safety of other road using is not compromised.
Looking through the windows gives much more information than from the mirrors, although the mirrors help with the positioning of the reversing vehicle in the road.