The hash is for content-based addressing. We're going to fasten those 60 truncated bits to a 4 bit tag that says this is a URI in the content-based storage. And then we can use this 8 byte value to refer to that URI twice, or fifty thousand times in our system even if the URI is a hundred bytes long - the only problem is collisions, so we need to be sure we notice if there is a collision and handle that correctly everywhere.