We are learning more and more about the bacteria in our gut and how they affect our entire body. An inbalance of healthy and unhealthy gut bacteria can lead to a host of stomach problems. In addition, antibiotics can sometimes cause problems by killing off healthy gut bacteria alongside the infection they are trying to treat. Taking probiotics preventatively with antibiotics can prevent this. See the article "6 Good Reasons to Consider Probiotic Supplements" from Prevention.com where I am featured discussing this.