I remember the ‘indicator rows’ from years ago when I did my blue light driving course. We were shown pictures/film of cars indicating, and asked to say what it meant; course we all said ‘they’re turning right/left/about to pull in’ etc etc.
Their answer was no, in fact all these pictures show, is that the car’s indicator light is working. Their point being, of course, in a transparently PA way, that you can’t presume the actions of a car/driver from their indicator lights. Due to all the reasons stated above by prev posters.
It means there’s a fine line between dithering unnecessarily at roundabouts etc, should you choose to presume everyone has left their indicators on by accident, or blindly charging out cos the car approaching is indicating to turn off/pull out.
The skill they’re trying to hammer home is getting that balance right; see their indicators, but read their manoeuvres etc. We were also taught to not indicate if no-one was visibly around (I still have no answer to what you’re meant to do if approaching a junction with limited view etc, they never did answer that 🤷♀️)