This is an Excellent question, and regardless of some people's misassumptions about the 'proper' place to post such a question, it is one that deserves an outlet since the public jira is not a forum, or a place to actually get answers.
Since you asked for what people 'think' (not what they know), this makes this question a good one.
Personally, I 'think' that if LL is purposely doing this, it is to stop or hinder the possibility of the auction process getting frozen by massive bids all during the final seconds of the auction (although considering the current land market, auctions are for the most part a joke). In previous auctions, there have been quite a few where bids that were submitted in the last few seconds would just get stuck, and there have also been numerous cases of multiple people appearing to win the same auction - or with bids that appear to have been placed at the same second for the same amount. If in fact there is a 'magic' number of seconds before the official close of bidding as opposed to it being semi-random, then there would certainly be a huge advantage to knowing what it was, but LL is also famous for allowing certain people to gain unfair advantage over others whether it be through inside knowledge or through technological exploits. More than likely, I 'think' such a delay would be to keep the bid freezes and other associated technical issues from occurring in the last seconds of bidding.