Most of my tours are bracketed by immutable dates, so I schedule zero days to not miss a meeting, a flight or something else.

Zero days are useful to deal with lousy weather (ex: miserably cold rainy day; gale force headwind), health issue (ex: upset stomach, flu, not so mild injuries consequences of a fall), and fascinating POIs (ex: museum, day hike).

I try to start a trip with a 1-day/week bank. But I never succeed. Typically half. Typically a few days left in the bank at the end of the trip.