As we move through the daily grind in which we live, our brains often function reactively. We have set "programs" that our minds use to deal with externally (from the environment) and internally (from our own bodies) generated stimuli. Meditation allows us a chance to escape the reactive trappings of our mind. This has been shown in many studies, and recently in Japanese Buddhist monks. What these studies show is an increase in frontal lobe activity during meditation. The frontal lobe is the newest portion of our brain, evolutionarily speaking. This portion of our brain allows us to discriminate, judge, and override more primal parts of our brain. In short, it is the part of our brain that makes us human--the part that is able to rise to a moral, philosophical standard, rather than being trapped by instinct and desire.
I bring all of this up because many believe that when meditating you are "doing nothing". In fact, you are improving the part of you that makes you uniquely yourself! If you wish to change yourself for the better it is crucial that you are able to choose the direction in which you wish to go, then override old programs to execute the new behavior. I know of no more powerful tool for this than to meditate. Read more on this subject on my Stress Reduction Page.
Read the study on Japanese monks.