Use Add Value when your client purchases additional value for their GC. You can optionally enter a comment.
If you're tracking points, you also use Add Value when a client earns more points. For example, you might decide that the $100 retail GC your client purchased is going to be added to their membership rewards tally, so you need to assign it 100 points. Simply assign that value in the Points field, skipping over the dollar fields and then save.