Begin nodes are required to tell the program that the scenario is going to start. Since the Begin node is always needed, the Storyboarder by default includes a begin node. This Begin Node is merely there show you and Modest3D where the scenario is starting.
A Begin node is also added to the beginning of all Templates . In this case you can add and remove parameters in the begin node to control what type of arguments the template requires.
Begin nodes are also used to define no name functions which are needed to use some list functions.