Many countries used to make their coins from silver, but that ended decades ago. You can still find these coins and they're easily recognized. Because of this, some include it in their holdings either for a long-term investment (Limited supply since it's no longer produced) or use as barter in an end of the world type scenario. While it certainly has the potential to appreciate in value over time, a large amount won't be used for barter.
Using US coins as an example, all dimes, quarters and half dollars made through 1964 are 90% silver. Where it gets confusing is that, to those who don't know the difference, dimes made of silver in the 1960s look the same as modern dimes with Roosevelt on them with no silver in them. The same goes for quarters with George Washington on them.
To make things more confusing, only Kennedy half dollars from 1964 were 90%, but from 1965-1970 they were made with 40% silver, and nothing valuable from 1971 forward.
This is why the majority of "junk" or 90% silver will not be used for barter. Your average person doesn't know the difference and is unlikely to be willing to learn when they're busy just trying to survive.
While you could use these for barter due to how unique they are, that same uniqueness also raises the possibility they could become highly collectible in the future, causing prices to rise dramatically.
You're more likely to have better success having extra ammo, liquor, beer, food, cigarettes, and other items to simply trade for what you need. Instances where precious metals would be useful for barter seem far and few. While it's nice to be prepared for what might happen, don't plan exclusively on this and have an exit strategy for this if you end up not needing it for barter.