While I agree that it is not necessary on a small hatchback, I do find it useful. Especially when reversing up to high kerbs not detected by the parking sensors. Also useful when parking as I can see the line of the parking bay behind me. So I know that I am perfectly positioned in the space.
There are sensors on our A3, but no camera. I'm surprised how much I miss it.
As brettblade says it is just reassuring to see into, what is normally, a blind spot.