This from the National Institutes of Health.
A prebiotic is "a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microflora that confers benefits upon host well being and health.". . . They are dietary fibers with a well-established positive impact on the intestinal microflora. Other health effects of prebiotics include prevention of diarrhoea or obstipation, modulation of the metabolism of the intestinal flora, cancer prevention, positive effects on lipid metabolism, stimulation of mineral adsorption and immunomodulatory properties, which are mediated by the intestinal microflora.
In other words, the fibers present in plants serve as food for beneficial bacteria, and by eating these foods you increase the population of these important organisms. Remember that you have 10 times as many bacteria in your body as you do cells. By feeding the gut flora, and altering specific populations, you are literally becoming a different person, whose major difference is a happier belly.
One more reason to choose plants.