There are two reaons to use it:
- If you are driving you expect that other cars use turn signal so you can decide what to do. Example, In a roundabout (circle way),changing lane or intersection , the turn signal shows the driver intentions there, so you can proceed or not. If someone do not use it you will need to wait because you don't know whatafuck the driver is going to do. So you should do the same, use turn signals to express your intention. It can create an accident in a road, when someone change lanes without using it. Imagine that I am behind a car in the right lane, I use my turn signal to the left to trespass the car, and in that moment the guy go the left lane without any warning...
- It is illegal to not use it in here. Failure to use a signal can result in fines and demerit points. If there is a cop nearby, he will surely stop you.