Global variables in DS are always defined at job level. Your jobs may become very complex with lots of workflows and dataflows. Have you ever tried to figure out where exactly in the job a certain variable was used?
An extension to the View Where Used functionality, added in SP6, has made this task a piece of cake.
Right-clicking on a Variable (or parameter) name now shows View Where Used in the pop-up menu.
Selecting the option View Where Used starts the Search functionality, opens the Search window and after a while displays the usage at the bottom.
You can open any object directly by double-clicking its name in the list.