Burning herbs to cleanse the energy has been a practice used for thousands of years. In many cultures, burning sacred herbs helps chase away negative and malicious energies from their homes. Smudging has also been used as means to purify and cleanse people as well.
Smoke symbolizes release and the herbs symbolize the energy you wish to release into your environment instead. Smudging is a spiritual act of cleansing, much like taking a bath to release your body of dirt and impuritues.