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]
It doesn't really matter how transparent it is, since, International Law has rather little ability to enforce its decisions.
As much as we try to develop rules and norms to constrain and moderate behavior by countries in the international community, at the end of the day, "the strong [still] do as they can and the weak [still] suffer what they must".
And yes, you're right that the actions and intentions are pretty transparent, with just some uncertainty surrounding the extent (ie, what reasoning will the PRC ultimately use to justify its claims, as it's also been quite ambiguous about them).
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...