Don't just think about currency exchanges as in buying and selling coins to hold. Think about blockchain services that use different tokens working together seamlessly: that's why decentralized exchange protocols are a game changer. For instance, a video may be uploaded on Paratii (PTI), transcoded using Livepeer (LPT) and stored on IPFS (FIL) - doing those operations without decentralized exchange would be a huge pain in the ass, but 0x (for example) makes it easy and practical.