My wife and I use the Take-a-look mirrors that mount the your glasses or helmet. I prefer the compact version. Wouldn't ride without them. Generally speaking, they serve the same function that mirrors on motorcycles or other motor vehicles provide. They're great for keeping track of traffic behind you as you need to move into the lane to avoid debris or hazards on the shoulder or avoid the door zone of cars parked along the road. One advantage of glasses or helmet mounted mirrors is that you can scan a wide area behind you by turning you head a small amount. Handlebar mounted mirrors don't generally provide the same field of view or the ability to scan.

I'm not sure it matters, but we're both also motorcyclist going way back over 40 years.