I've used it when backing into spots and parallel parking. I've also used it when pulling through a spot to make sure I've pulled through enough.
Since I don't back into spots that often, my primary use is to make sure there is nothing behind me when I go to back out of a spot or my driveway. I look all around me though not just at the camera. It's just another view point.
As part of backing out, I've also used it to see how close I am to the cars that might be in the row behind me. That's especially useful in tight parking lots.
But whenever I use it, I'm also continually rotating between looking to my left and behind, rear view mirror, right and behind, and back up camera.
When I first got the camera, I had someone stand say 6-12 inches away from my car so I got a good feel what that looked like in my camera so I'd know when I was getting too close to something.
My husband's new car has a back-up camera plus sensors in the front and rear that beep when you get close to something. You can adjust the sensitivity as well. I think it's great, especially if you don't have a good visual feel for how close the side front bumper may be to something. He hates them though and turned them off.