Why would you want to use a rear-view mirror?

That last reason is the most common one. In response to that, you might wonder: Why not just turn your head and look behind when it's time to make a move?

Well, before you act, you might have to check backwards multiple times. With a mirror, you turn back only after you decide it's safe to make a move, to make sure you did not fail to see someone. There are several reasons why it's better to minimize turning back:

Note: before looking back, with or without a mirror, you should first look ahead for things that could become dangerous while you're checking behind you. If you see something threatening, you might have to defer looking back.

You can buy mirrors that attach to the end of your handlebar and stick outward, like a car's driver-side mirror. I have not tried using one, but I've heard that these have a couple problems:

You can also buy mirrors that attach to your helmet. I've never tried mounting a mirror on my helmet. I'm guessing that doing so would eliminate the two problems just mentioned. But I wonder if helmets are loose enough that the vibration problem still exists.

I like to use a mirror that mounts on my sunglasses or safety goggles, though it can also mount on a helmet. It's the Take-A-Look brand of cyclist mirror.

Eye glasses with attached mirror, inside view Eye glasses with attached mirror, viewed from above

Some people worry that the mirror would create a problem by blocking the view to the left. It does block the view a little, but I can turn my head slightly left or dip it down when I need to.

One issue with my mirror is that it does not work as well with my road bike as it does with my comfort bike. There are two reasons for this:

This means that I should not use the mirror, backpack, and road bike at the same time.

I installed my mirror on a pair of wrap-around sunglasses.

Sunglasses with attached mirror, viewed from above

The mirror is closer to my eye than it is when attached to those goggles shown previously. When I first started using this setup, I had to strain my left eye to see what's behind me, but I have since adapted somehow.

The mirror is easy to remove when I leave the bike and want to keep using those sunglasses. Also, it's only a minor nuisance to switch that mirror between sunglasses and clear goggles as the amount of daylight changes.