A more serious (but less Buddhist) answer to Monkey's question would be, "Because if you learn to think about nothing, then you are better able to think about one thing very well."
There are several things I found interesting about this particular finding.
First, it shows that the brain is not some sort of static organ. It can be trained, deliberately, to do certain things. Not exactly a new finding, but futher confirmation of what is already known. What you think about, and how, will influence your future ability to think. When one contemplates the number of people who spent an hour watching "Desparate Housewives" last night, that's a scary thought.
Second, it provides evidence that the perceived mental changes that come about as a result of Vipassana meditation are very real. It isn't just some sort of placebo effect. Your brain actually changes.
Third, many religious people of very different religious tradtions that involve prayer, meditation, or ritual will insist that their chosen rituatls actually change the way they think. This study demonstrates that they are probably right. Now, many of those people think that what happens is the "Spirit of God" or something like that enters into them, and it is His spirit that changes them. This study would indicate that, if that is the case, God is busy rearranging brain cells. Nonbelievers might suggest that the perceived "Spirit of God" might just be a change in your brain.