The advantage of using a global list instead of a hard-coded list in the form:
Change once, change everywhere. With hard coded combo boxes, you have to fix the values on each format.
if you need to support localization, it's designed to work with global lists.
As to value list/display list:
If there isn't a difference between the value list and display list, you can populate only one of the values on the form (there is no need to populate both). I usually use the value list, but you can use either the value list OR the display list. SM will accept whatever value is selected.