General purpose and usage of the type.
Adding information to a relation usually takes a separate content item (and presumably content type) to use as storage between on each relation. No need for such complex (and of course complicated to wrap once head around).
The list can also be used as a glorified regular relation list (by not adding any attributes) and improves on the UX by using drag'n'drop functionality to sort relations.
Groups can be used to replace multiple relation lists and trim down the content (although this might not be useful in every use case)
When you take a look at [https://www.intense-programming.com] the menu (top left/right) as well as the footer are configured using a single content field of this type withing the root landing page.
The grouping functionality is used to add elements to either the Left side menu, Right side menu or Footer links.
Attributes are used to control the dropdown on buttons (Display children, Children count, Order by).