From what I read he just wanted to use his wealth to invest in the local economy and as a bit of a PR stunt for his chain of Sushi restaurants.

Bloomberg's coverage of it was good: http://www.bloomberg.com/news/2012-01-06/the-sushi-auction-that-cheered-japan-the-ticker.html

I mostly just question the ethics of using the fishing of an endangered fish for consumption as a PR move, but Japan feels differently about some things than other parts of the world and that's their right.

Either way, if you ever find yourself in Japan, definitely swing by the Tsukiji fish market where the sale of this fish took place. The market itself is a crazy sight to behold, there's nothing else like it in the world.