I use it to get right in the middle of the space and make sure I don't hit anything.
Obviously, I still look left and right for cross-traffic.
For a curb, get the curb just out of screen and the bumper will line up right to the edge of the curb. For a car, stop backing up at the first red line, but you should park with him at the first blue line, or he's gonna tap your bumper to get out (assuming he can even get out).