Many people used to ask me this question, read the below article published @SignageInfo.com

TVs are designed for at home broadcasting, while digital displays are designed for narrowcasting, i.e. reaching your target audience. Internally a commercial display is different with an integrated media player and no requirement for a TV tuner. Commercial content can be displayed directly from an external media player or via memory storage devices (USB, Compact Flash, SD card). On a TV if someone doesn’t like the content being displayed, they change the channel, a TV is designed to be able to change channels or input devices easily – you don’t want this with a commercial display and so these have lockable compartments and no visible control buttons or panels. Additionally comprehensive digital signage solutions include content scheduling software to maximise versatility and provide targeted content through the use of playlists for different times of the day.

Read more at http://www.signageinfo.com/2011/02/digital-signage-cant-i-just-use-a-tv/