Who are they indicationg to? The answer is no one: it is just a habit they have: change lanes so they'll indicate rather than take good obs.
Non sequitur. Just because they have indicated when you think they shouldn't does not mean they haven't looked. They could, quite clearly, have looked, seen nothing but have indicated just to be sure.
As you you yourself say you're not perfect, so for that small percentage of the times you have made a mistake, indicate. It might prevent an accident. What harm can it do?