The turn signal or the blinker is just a good idea. I don’t know why people don’t use them. It’s for your own safety and the safety of other drivers. Using your turn signal can just be considerate sometimes.
Consider this scenario. You’re sitting at an intersection wanting to turn left, but there’s an oncoming car from the left heading towards you. If you knew that car was going to make a right turn then you could go ahead and move forward and make your left turn. However, if the other car doesn’t put on their turn signal then all you can do is wait and wait and wait until they make their turn. Then you can mutter some obscenities and go on.
I recommend to get in the habit of using your turn signal even if no one is around. Sometimes people are around but you don’t even realize it.
What’s the harm of putting on your turn signal every time you plan to make a turn?
Unless however, you ran out of blinker fluid. This video explains much more.