It addresses something in the original books that didn't seem quite right, explains it in terms of the culture and history of Earthsea, then changes it. The things that happen here reverberate backwards- like the slamming of a door on an echoey corridor.
There are moments of considerable beauty. And look, a fantasy novel in which there are no battles or killings!
As a book in its own right it isn't very good. It couldn't exist without its predecessors- and the leading characters (there are a lot of them to keep track of) are all rather insubstantial and Mary Sueish. Still, I'm happy to have it.