Obstacles
I may be wrong, but I seem to remember that there is actually a regulation that says that if a PAPI or other visual approach indicator installed, that you must follow it. Precisely because they are not all 3 degrees!
I know of at least two airports, where not following the visual approach indicator can have you greeting hi tension wires in one case, and dangerously close to the tops of trees on an island in a river just off the threshold. Pilots focus on the landing zone and "forget" about the bits sticking up....
Above and steeper is fine of course.....
Oh.......a third airport comes to mind, where the PAPI (and displaced threshold) keep you away from being too close to the elevated commuter train station. Breaking the modern stabilised approach philosophy, I follow the PAPI on that one too until past the airport fence, then increase my rate of descent, decrease power, flare adding a bit of power, and land just past the threshold.
Ono