Y did the rockects trade rudy gay?

I have no idea why the Rockets would trade Rudy Gay and Stromile Swift to Memphis for Shane Battier. When I heard about the trade, I was just stunned. Why would you trade a promising rookie and a good athletic power forward like Swift for Battier??? Battier is an average NBA player-he doesn't score much, doesn't rebound much and is nothing special. I really don't understand this trade and I don't know why the Rockets didn't keep Gay because he's going to be a good player in the league.