Typically, when you would use the ContentPresenter directly is within the ControlTemplate of a ContentControl to mark where the content is to be added.
Every ContentControl type has a ContentPresenter within its ControlTemplate. The ContentPresenter is responsible for the UI generation. For example, when you apply a DataTemplate to an ItemsControl such as a ListBox, behind the scenes it is actually the ContentPresenter within the ListBoxItem that creates the UI of each item. For more information about that particular example, see ItemTemplate.