I believe most people do not adjust their left and right outside mirrors properly. If you're old-school and adjust them to see a bit of the sides of your car, you're doing it wrong.
Here is a good description of proper mirror "tuning". I've been doing it this way for years. Works great. Note: I do not use this method on larger (longer) vehicles where a visual of your off-tracking is needed.
https://www.scottsfortcollinsauto.com/tips-for-adjusting-your-car-mirrors-to-maximize-visibility/
I've always done it that way. My parents didn't, however. Of course, when they started driving (and even when I started driving) side mirrors were optional and, even then, usually found only on the driver's side. Both of my parents referred to the side mirror as "the side rear-view mirror," and that's how they used it. Which is, as the article says, redundant.
The problem today is that in SUVs and vans (particularly), the rear corner posts and hatch frames are so fat that the part of the rear window you actually see through is comparatively small. IMHO the article is organized poorly, because it says to adjust the side mirrors to cover the blind spots, but it doesn't tell how to adjust the rear-view mirror until after that. I adjust the rear view mirror first, than I adjust the side mirrors to cover the blind spots.
Then I go up on a highway and I check. I want to be certain that I can continuously track an overtaking vehicle from where it passes out of view in the rear-view mirror through the range of the side mirror until I can see it beside me with my own peripheral vision.
And that's getting more difficult with newer vehicles, mostly because of the shrinking aperture of the rear windows.
Also -- I didn't realize just how much I use and rely on the mirrors until the adhesive hold the rear-view mirror to the windshield failed in my late wife's car. I had to drive it with no (interior) rear-view mirror for a couple of days until I could get an epoxy kit and re-mount the little tab to mirror fastens to. It was VERY disconcerting and uncomfortable. I was constantly aware that I was "flying blind" and that I didn't have a grasp of what was happening around me.