Signals are one of the most incorrectly used instruments on a car and they are often "robotically used". When I say robotically used, I mean used as part of a process for doing a junction or a turn because historically, that's what most driving instructors made you do when you learned to drive. Most people have heard of mirror - signal - manoeuvre as this is the old school method taught to learner drivers. This creates the assumption that mirrors and signals must be done together and this is what I refer to as "robotic driving".
Generally, when someone does a signal robotically, they haven't done their observations correctly and fully as if they had, they should have used the observational part as a reason to work out if they need to signal.
Rule 103 of the Highway Code says the following:
Signals warn and inform other road users, including pedestrians, of your intended actions. You should always
give clear signals in plenty of time, having checked it is not misleading to signal at that time
use them to advise other road users before changing course or direction, stopping or moving off
cancel them after use
make sure your signals will not confuse others. If, for instance, you want to stop after a side road, do not signal until you are passing the road. If you signal earlier it may give the impression that you intend to turn into the road. Your brake lights will warn traffic behind you that you are slowing down
use an arm signal to emphasise or reinforce your signal if necessary. Remember that signalling does not give you priority.
If we read this rule correctly, it says that a signal should be used to inform other road users of your intended actions. This means that the observational part of the moving off procedure has to be to work out if a signal is required.
There is an argument to have about just using the signals anyway but this normally means that the observational part of moving off hasn't been given as much attention as it could have and could lead to misleading other road users, which is also stated in rule 103.
I tend to find that my teaching methods are different to most driving instructors as I like to stay as close to the highway code as possible and I prefer to make my students think about their actions rather than do something purely for the sake of doing it. This is likely down to my advanced training but nonetheless, it does work. For example, when moving off from the side of the road, I will always encourage my students to only signal if its required and the same goes to stopping on the side of the road.
The historical process of Mirrors Signal Manoeuvre is a flawed process.