The point was to let us know that Nikki has a hot little... posterior.
As for lack of plot advancement, well we had a run of some good eps so a pointless filler ep was about due. I didn't mind too much myself, it reminded me of the awesomeness of the pilot episode and also how glad I am to be rid of Shannon. Actually tbh I liked the episode, the whole buried alive thing was ultra creepy. I laughed at Sawyer saying "who the hell is Nikki" straight after she "died".
However, I am disappointed at not seeing and earlier Xerxes Paolo flash back