If you accept that Blockchain is useful for record keeping (and the innovation is that it's useful for trustless record keeping) then crypto currencies are usef

If you accept that Blockchain is useful for record keeping (and the innovation is that it's useful for trustless record keeping) then crypto currencies are useful as part of that ecosystem. Smart contracts for example - very useful to have crypto currencies that can be added to a contract or be given to a contract. Also, having a currency/credit is useful to manage economics (e.g. it costs "gas" credit to run smart contracts).