I use them every time I turn. I use them for every lane change,even on an empty street. I do not want to put up with a smart ass cop giving me a hard time for something that involves a flip of the finger. On an empty street you are very visable. If a cop is hiding and bored ,he can pull you over.I try not to give them any excuse.
Part of that is knowing what people are going to do - in advance. I’m not saying you judge other drivers’ actions solely on their turn signals; that would be daft. But neither do you wait for them to start doing something before taking responsive action. If someone ahead of me signals, I slow down. I’d have maintained the same safe gap anyway, allowing me to avoid hitting them even if they didn’t signal. It makes my drive a lot more pleasant and less stressful.
Do you mean that you previously never heard of a priest doing anything wrong? 
You don’t. And they shouldn’t. One of my pet peeves is turning and crossing that dashed curved line that guides the turning lanes in the turn. When in the rightmost left-turn lane, you cannot legally change lanes during the turn. If your rightmost left-turn lane is left-turn-or-straight, then you can signal a left turn or not signal to indicate driving straight. The people in the leftmost straight lane cannot legally change lanes into your straight lane when going through the intersection.
Whether or not someone signals or does not signal, you correlate their signal or lack of a signal with, as you say, what they are actually doing. I also correlate my actions with what they have been doing. If a signal comes on or goes off, I have a little bit more information on which to base my prophecy of their upcoming endeavours. If they wish to forestall my prognostication then I am doomed. Doomed I Say!
The signal is just a little piece of additional information, and “so shines a good deed in a weary world.”