Handwashing is one of the most easy and effective ways to prevent the spread of bacteria, viruses, and other microorganisms that cause disease. Rinsing only with water does not rid your hands completely of these microbes. Only scrubbing firmly with soap and water will do the trick!
Especially as flu and cold season come around, it's important to wash your hands thoroughly for your own health and for the health of others. Consider going in for an extra hand wash after shaking hands, being around sick people, or after coughing/sneezing!