We've talked about the distance a keeper plays off the line with a shot from a set spot. That all changes with a little thing called a pass. A shot from the endline isn't necessarily dangerous, but a pass to a player at the penalty spot from the endline is. Similarly, an opponent with the ball and a defender on him is not likely to be as dangerous as the opponent's open teammate. Modern hockey has shown that a keeper can't just be a goalkeeper, he needs to be a circle defender.
Angles incorporate all of that.
Angles build on knowing where you are in the circle. We've talked a lot about keepers being a circle defender and not just making plays of the ball on the goal line. There's not a whole lot of advantage to a keeper coming off his line, if the result is that he's going to concede a goal on the pass because he doesn't know where he is. It's important to know where you are in the goal from landmarks around the field. Use landmarks like the opposite goal, the arcs of the circle, the 25 at the sideline and long hit marks to find your bearings.