well, it's really generate a random point and then take it's angle AND distance from the origin.
It's not just a random angle
it's not just a random distance frome the origin
it's not event a random angle and a random distance, 'cause you have to stay within the rectangle boundaries. If you take a random angle and then a random distance from the origin (from 0 to how much?), you can't be sure you are within the rectangle. Angle and distance are linked to discribe a specific point.
So it's really a random point you convert into distance and angle from origin -> rotate -> translate.