Ladies and gentlemen of the driving persuasion, one of the safety features of your carriage is the turn signal. A simple indicator presents a…well, a signal to other drivers about your intention to turn left or right, as the case may be.
You may think that such information is unnecessary for other drivers; after all, you know your own intentions, and that's all that matters. Alas, as many have learned the hard way, this is not so. The signal allows other drivers to react to your impending turn or lane change in a way that safely accommodates both vehicles.* As safe a driver as we both know you personally are, it is just this side of possibility that you may seek a lane change entirely unaware of another vehicle occupying the same spot at the same time as the space you would choose for yourself. Should you signal your intentions, and should the other driver see this and react appropriately, the two of you may avoid an unwanted and potentially unpleasant exchange of views on the side of the road.
On city streets, the turn signal serves yet another useful purpose, which is to (ahem) signal to pedestrians crossing the street that you and your vehicle desire to change direction, and those pedestrians may usefully incorporate that knowledge in their own behavior.
All in all, the signal is a useful, but sadly underused, part of your vehicle. Thank you for your attention in this matter.
* Of course, some drivers take the signal on another vehicle as a direction to close up any gap in the traffic flow so as to not allow the signaling vehicle a space. This note acknowledges such behavior and designates it as Impolite. We live in an imperfect world.