Basically Stub activities are used to enable your application to extend and override standard work object processing. These doesn't have any steps in it and are known as extension points.
For example, If you see Work-.New Activity calls Work-.createWorkPage which in turn calls Work-.NewDefaults stub activity.
NewDefaults called from Work-.New and Work-.NewCovered, this extension point activity allows the user to customize or extend the steps that are performed while creating new work page for the new harness.
For more information go through this documentation about stub activities and explore in the application.
Hope this might give you broader view about stub activities.