This one is sort of very specific and niche, but as a caregiver of someone with an ostomy, you can take a roll of this disposable plastic produce bags from the grocery store. Terrible for use as a produce bag because they're plastic and disposable and whatnot, but for bag empties when you need to use something disposable no matter what? They're invaluable.
Also doggy poop bags. Great for that. Just grab one of the small almost empty rolls, and tell the cashier "I'mma take these" and no one gives a shit
Huh, I've always just bought small trashbags, never crossed my mind to just grab the whole roll of those produce bags. Still might not, feels wrong to just take them. :)