Usually the editor won't start scrolling the text unless you move the cursor to the last visible line and press down (or first visible and press up).
Scrolling margin means that there will always be X lines visible under or above cursor (except at the beginning of the file). So if the cursor is on the (X+1) line from the bottom of the screen, pressing down will scroll the text and move the cursor.
This option would be very convinient, because the user will always have a better context of where in the code (s)he is now.
Customer support service by UserEcho