The addPluginRollouts() is used within scripted plug-in create tools, specifically for level 1 plug-ins that don't create instances of themselves in the scene but might create other objects within their create tool. This method lets you install rollouts for a specified object in the Create panel, typically one of the objects being created in the create tool, so that its parameters can be adjusted directly during creation.
In the following example, a variation of the three light mouse tool example presented in Mouse Tool Clauses is set into permanent, editable classes. It defines two new classes. The helper class lightMaster is used as a master object for the light system. The light class threeLights is what you create to layout the initial system. It also creates a lightMaster and sets its parameters to point at the lights. The lightMaster rollout allows subsequent editing of the system in the Modifier panel, and storing the lights in a parameter block makes the setup persistent over scene saves and loads.