The first car in my life was Mom's '53 Chevy. (Coincidentally, I'm also a '53 model.) Mom's car had a 3-on-the-column manual transmission. A key was optional (you could start it without a key, or you could lock the ignition with a key). There wasn't much plastic used on that car... the shifter had an ivory-colored plastic knob; that's about it. And... it didn't have turn signals, other than hanging your arm out the window.
Turn signals were mandated on all cars shortly after - possibly the very next model year. Well over 50 years ago.
The first car with blinky-light turn signals was the '38 Buick, and in 1940, Buick added an automatic shutoff mechanism. (Before that, as early as 1909, a few of the more upscale model cars had little semaphore-like mechanical pointers called "trafficators," that the driver could activate to signal intent. But the mechanism was never very reliable.) So, more than 100 years ago, some folks thought that signaling turn-intent would be advantageous from either a safety or traffic-flow perspective.
There's a law that mandates use of turn signals. (This would come as a surprise to many.) In Idaho, it states, "A signal of intention to turn or move right or left when required shall be given continuously to warn other traffic. On controlled-access highways and before turning from a parked position, the signal shall be given continuously for not less than five (5) seconds and, in all other instances, for not less than the last one hundred (100) feet traveled by the vehicle before turning."
"What about bikes?" you ask. In Idaho (and in most states, I imagine), the law states, "A signal of intention to turn right or left shall be given during not less than the last one hundred (100) feet traveled by the bicycle before turning, provided that a signal by hand and arm need not be given if the hand is needed in the control or operation of the bicycle." Some people like to use the dedicated left-arm signals - you know, point straight out for left turn and point up for right turn. I prefer the no-brainer - I point left with left arm to signal a left turn, and point right with right arm to signal right turn. Either is legal. (I like no-brainer; the left-arm thing requires both signaler and other roadgoers to do more mental processing, IMO.)
Almost every day, I find myself wondering if turn signals, and a turn signal law, are obsolete. After all, they came into being back when people were able to concentrate more on their driving. Back then, they didn't have to hold a cellphone in one hand and that big Starbucks cup in the other hand. Those old fogey lawmakers couldn't have foreseen a time when driving would be an unplasant distraction from all the other tasks at hand. Turn signals are hard!!!