Using a shoe to hammer a nail into drywall is one way to improvise when you don't have the right tool on hand, but in a typical day, we use our fingernails as openers, knives, scrapers and pickers, just to name a few. Peeling off a price tag, scraping food from a plate and doing the thumbnail push to get the gunk out of keyboard crevices is kind of convenient in a moment of need, but it does a number on nails. And, even though it is one of the more common uses, most of the time nails are not so great at getting to the food between teeth; they often just get stuck there and break or tear.
Each force and bend of a nail causes tiny fractures at the tips and can lead to breakage, so having tools on hand for smaller-scale jobs might help avoid nail breakage. Tiny, keychain Swiss army knives, small toothpick containers or even desk-drawer letter openers or nail files can do the same or a better job than your nails and will help spare them.