Add To Calendar (A2C) is small utility that once launched let's you start adding new entry in your favourite calendar application. Once A2C is launched you should see "Add New Event" dialog of your calendar application (or Calendar chooser if you got more apps installed) and that's what is designed for. It's extremely useful if you want to create shortcut to add new event but the application you want to create this shortcut with (i.e. Widgetsoid that puts that in notification area) only allows you to select the application. But selecting Calendar won't work as you want to add new event, not see your calendar details or agenda overview. Or, you can have Add To Calendar on your Launcher to easily and quickly create new calendar entries.