Beginners and intermediates will spend most of their time on their practice chanter because they are learning the basics of the instrument and tunes. Beginners don’t even have pipes yet. At that stage they are learning their first tunes and working on their technique, and all of that is best done on the practice chanter.
Once you’re past that stage, you should play your practice chanter when you have a specific reason for memorizing a tune, improving technique, or trying to refine
the expression of a tune. Too many pipers default to their practice chanter because it’s easier than pipes and they fool themselves into thinking they are doing more good for their piping than they actually are.