From what I can gather, it is supposed to specify how much you're "looking down" on a scene, as opposed to looking across. It only affects how the y distance is measured.
As the pic angle approaches 90, y the distance between things will approach infinity.
So 0 would be straight down (and dy would be unchanged), and 89 would be nearly straight horizontal. Looking nearly horizontally at something (89 degrees), stuff that is 1 pixel y away from another thing would actually be 57 units away (per GetDistance kernel).