This is mostly strategic: China is effectively hemmed in by Japan, Korea, Taiwan, and a bunch of SE Asian countries. They simply don't have easy access to the ocean from a military point of view (they are easily cut off during a war). They are trying to claim the whole of the SCS to fortify it and ensure open routes in case of a war.

It does seem awfully unfair looking at a map though, basically China's claim comes without 10 miles of other country land borders. Nobody but the Chinese would think these claims are fair.