From the FCC's website:

Why Are Broadcast TV Stations Switching to All-Digital?
Congress mandated the conversion to all-digital television broadcasting, also known as the digital television (DTV) transition, because all-digital broadcasting will free up frequencies for public safety communications (such as police, fire, and emergency rescue). Also, digital is a more efficient transmission technology that allows broadcast stations to offer improved picture and sound quality, as well as offer more programming options for consumers through multiple broadcast streams (multicasting). In addition, some of the freed up frequencies will be used for advanced commercial wireless services for consumers.