I've just discovered this setting for the first time. How long have we had this?! I'm guessing forever, lol.
It allows you to use standard views (View > Standard Views) to look at your building model at right angles when the plan is rotated.
To trigger this setting: say you have a site plan with north up the screen and your building is it at an angle. You rotate the plan so that your building is at right angles to the screen and then activate one of the standard views for the first time.