The whole purpose of this system is to ensure that the person over-taking you never falls in your blind spot i.e. the side-view mirror that is on the passenger’s side.
Technically speaking the indicator should be given at least 50 meters before taking the actual turn, but in our case it’s given while turning. Even in the case of lane changing, drivers need to give the person behind them at least a few moments to notice the indicator, but again the signal is given and the turn is taken immediately so the guy behind him thinks “Oh you don’t say?, if you didn’t give the indicator while you turned, I would never have noticed!!”. This is often the cause of that tyre screeching noise caused by slamming the brakes, when a driver is caught unaware.