Overhead light comes from behind you and drops your own shadow onto the thing you are trying to look at. A desk lamp exists to come from the side; a gooseneck exists so you get to decide which side.
The arm holds position wherever you bend it rather than sagging back down over the course of an hour. Brightness and colour temperature both adjust — cooler while you are working, warmer at night when you would rather not be told it is the middle of the day.
