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.
The Problem
Many people are unaware that older coins were made of silver or gold, instead of lesser value base metals. While some do know this, many in this group don't know the details of which coins were silver or gold and which weren't.
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.
Exceptions
There are some exceptions to this, as some fractional sovereign silver is unique and old enough that it can't be mistaken for modern coins. Using US coins as an example again, older quarters have a figure of Liberty standing up, older half dollars have Liberty walking or Benjamin Franklin on them, older dimes have Liberty in a winged hat on them, and even older dimes, quarters and half dollars depict Liberty wearing a hat with an Olympian-style wreath. These designs aren't on any newer coins and are very iconic and unique.
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.