Why do herbalists categorize herbs by 'Actions' and 'Energetics'?

Knowing whether an herb is warming and drying (like Ginger or Thyme) versus cooling and moistening (like Marshmallow Root or Linden) prevents you from matching a drying herb to an already parched, dry constitution.

Can one herb belong to more than one tradition?

Yes! For example, German Chamomile is both a Calming Nervine (when steeped lightly for 5 minutes) and a Digestive Bitter/Carminative (when steeped longer for 12 minutes).