This increases visibility and is particularly useful when traversing large objects during off-road driving, saving you from having to stick your head out of the window to make sure that you aren't going to hit any large, low down objects that would otherwise be difficult to see!
It's also really useful when parking, allowing you to see the curb and pavement without having to adjust your main mirror. The small Assist Mirror is manually adjustable, so once you are sitting comfortably you can position it perfectly to suit your line of vision.