A big part of this is also "creating facts on the ground" that align as closely as possible with the PRC's "9-dash line"[1], especially prior to a decision being made in the Philippines v. China case before the Permanent Court of Arbitration. Those facts also provide much better power projection capabilities to the PLAN (the PRC's Navy).

Development of the islands actually accelerated dramatically when the case was filed, and I suspect the PRC wanted to get the largest and most obvious work done before that case concludes, since, although the PRC doesn't recognize the court's jurisdiction, past cases where other countries have acted with similar disregard suggest the optics surrounding it still matters.[2]

[1] See: http://www.bbc.com/news/world-asia-pacific-13748349 and https://en.wikipedia.org/wiki/Nine-dotted_line

[2] For example: http://systemchangenotclimatechange.org/article/caving-inter...