Here's my 2¢ for what it's worth.
I have a mirror on road helmet I wear when riding my road bike. I want to be able to see who or what is behind me.
I have a mirror on my "commuting" helmet and a handlebar mounted mirror on my commuter bike. I want to be able to see who or what is behind me.
It's as simple as that. I don't race, I ride for fun exercise, fitness, pleasure, fresh air ...
__________________
The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.