They are trying to better control the area (and likely trying to protect it, as well, as it impacts their own interests economically). Note that approximately $5 trillion of global trade, goods, and commerce flow through those waters every year.
It's also a dominance and power play since the Philippines, Taiwan, Malaysia, Vietnam and Brunei also have overlapping claims in the waters. China is edging them out a bit and reminding them who's in charge.
Taiwan has done some build up work as well, but nothing at the scale or pace we've seen from China.
Some before and after photos: http://www.marketwatch.com/story/chinas ... 2015-10-27
EDIT: I realize this doesn't tie directly to Japan as asked in the OP. Happy to split this off if the request to do so is made.