Coins produced in certain years have higher Silver content than others. You will typically find that the older coins have a higher Silver content like the 1878-1921 Morgan Silver dollar which is 90% pure Silver. Compare that coin to the Kennedy Silver Half Dollar which is 40% Silver.
Some people are purchasing as much of these coins as they can to hedge against a possible hyperinflation environment in the coming years. These people believe that they will one day be able to use these coins to purchase groceries at the market as the precious metal inside (Silver) will have purchasing power compared to the US Dollar. (Which is a piece of paper with green ink on it)