I carry a bag when I’m in the field. I carry a bag when I’m running errands around town. I carry a bag more or less whenever I leave the house for anything more than to walk the dog or take my evening stroll. Ideally, so that I need not worry about being without some item of my essential kit as a result of switching between different bags for different activities, I very much prefer the bag I carry for all these diverse purposes to be the same bag.