Right now, when the Constrain Angle setting is at its default value of 0°, you can hold down the Shift key to move an object along the 0°, 45°, 90°, 135°, (etc) axes. This is fine.

But if you change the constrain angle to something else, say 30° (as someone making isometric illustrations would be likely to do), then the set of constrain angles changes to 30°, 75°, 120°, 165°, etc. This is not useful. It would make much more sense for the set of constrain angles for a setting of 30° to be 0°, 30°, 60°, 90°, 120°, 150°, etc.

Similarly, the set of constrain angles for a setting of 50° should be 0°, 50°, 100°, 150°, etc.