Do Cryptocurrencies provide value and do something that country backed currencies can't? They do have their own advantages and disadvantages, but it purely boils down to how many people trade in it.
Take gold for example, people believe that it has value and trade in it. The price of gold is decided on the basis of market forces of demand and supply. Similarly, a cryptocurrency's worth is decided by the demand for it. If enough people trade in it, then it can be used as a bartering currency. Heck, even if enough people decide to use eagle's feathers as a token to buy and sell goods in exchange for, it is valid currency for those set of people. The larger the set, the more accepted the currency.