The sarter is a web application that has been added to create and start all the portals (i.e. portal containers) at the same time when all the other web applications have already been started. In fact all other web applications can potentially defined several things at startup such as skins, javascripts, google gadgets and configuration files, thus the loading order is important as we can redefine skins or configuration files or a javascript from a web application 1 could depend on another javascript from a web application 2 so if the web application 2 is loaded after the web application 1, we will get errors in the merged javascript file.

If a PortalContainerDefinition has been defined, the loading order will be the order that has been used to define the list of dependency. And if you defined a PortalContainerDefinition you need to deploy the starter otherwise the loading order will be the default one (i.e. the loading order of the Application Server)

So if you need to customize your portal by adding a new extension and/or a new portal, you need to defined the related PortalContainerDefinitions so you need to deploy also the starter. Otherwise, you don't need to define any PortalContainerDefinition and to deploy the starter.