When wouldn't you?

When I go grocery shopping, I'm buying probably 20 pounds of veggies, meat, milk, cheese, eggs, etc. along with bulky items like tortilla chips. If you use the woven plastic bags that you carry in your hands (like the old plastic bags but twice the size), you need 4 -- two for each hand.

There are also the jumbo super-heavy bags you get from e.g. FreshDirect where you only need one and you sling it over your shoulder, but those things are huge even folded up and I don't want to be carrying around one of those regularly. Folded, they're thicker than my laptop...