Well, the claim is that a decentralised currency is harder to manipulate by any one organisation or bank, since the control over the currency is spread on so many hands, meaning all the miners – and anyone can mine (at least if you can afford a GPU these days). That prevents banks or states from freezing your assets, for instance, which actually has happened, with Julian Assange and WikiLeaks being a prominent example. Such a asset-freezing simply isn’t possible to do to most cryptocurrencies.