Let me start this off by saying that I have played baseball my whole life, and actually played in college (not for a club team.. the actual team) and I can't think of why this method of training is preferred.
A lot of younger, developing players (middle school, high school, & college) prefer to hit with woodbats during their winter training, rather than the bats that they are going to be using during the regular season.
Now, the only reason why I have heard as to why this is, is because a woodbat has a smaller 'sweet spot' if you will, or in other words, it doesn't have as much forgiveness as a metal / aluminum bat. So in translation, if you can make solid contact with a woodbat, then you definitely can make solid contact with a metal / aluminum bat, especially with the amount of forgiveness that those bats give.
My question to that is.. why would you train with a bat that you only use a few months out of the year and you don't use in games? Shouldn't you train with what you are going to use in games, so that you can build large amounts of confidence with that piece of equipment?