It works the same way like moving an object by pressing one of the arrow keys to lock it to a specific axis.
The simple advantage of this would be in allowing objects to be neatly aligned.
So if i wanted to present a table to someone from a forward perspective view but wanted the angle to be slightly more steep to show more of the top surface of the table while keeping the table perfectly straight so that the horizontal edges are perfectly aligned with the horizontal axis then i would be able to do that with ease.
With the way SU works now that would not be feasible without some time consuming workaround.
Another advantage would be to get a clean and precise 360 degree orbit around the vertical axis of the center of an object or scene.
Certainly such a feature would be handy and not too difficult to implement.
It also doesn’t cause SU to become bloated since a person is simply not obliged to use the arrow keys which are in themselves not being used anyways at present with SU as there is also no additional buttons to take up space.
To take it even further with symmetry why not have orbital snapping like way there is rotational snapping. Perhaps you can activate the snapping by hitting the alt key while orbiting with the mouse. So if you have the forward perspective view for the table then you can perfectly tilt it upwards around the horizontal axis by exactly 45 degrees.
Such a feature if implement in SU should be part of SU itself and not an extension since its an elementary function.