Why do I need an antenna as well as broadband to access Freeview On Demand?

Freeview On Demand is an integration of live broadcast TV and on demand TV, delivered using the HbbTV standard. 

An antenna is required to pick up the live TV signal, through which the launcher or initialiser for the Freeview On Demand application is transmitted; while broadband is required for your on demand viewing and delivery of listings data.

Once you have an antenna and broadband connected to your Smart TV or box with Freeview On Demand, you can easily switch between Live TV and On Demand using just your remote control and from the comfort of your couch, without having to adjust your set up or go into an apps menu.

See here to find out more about Freeview On Demand.