Pop Up Message is useful when you want to warn users that an unusual value has been entered.  

  1. For example, if you put a pop up Conditional Action on your Height field for > 85 Inches.  A pop up message confirms "An unusual value in the height field has been entered.  Are you sure of your entry?"  This will not fire system query but instead just ask entry person to confirm data.  Then can just move on and no action will be required if indeed that person was very tall.
  2. Or if user enters data backward as in example below is another good example of when a pop up message can be used.