The add the ability to group pages on the left nav panel without needing to stick them into different interfaces.
How does is solve the user problems?
Once an interface has beyond 8 pages, it is too large on the left nav bar and pages need to be grouped. Grouping pages allows users to create a much better an organized user experience.
Note that this grouping is currently possible by putting pages into separate interfaces, however this causes other problems since record details pages cannot be shared between interfaces (and cannot be deleted), making it extremely difficult to manage syncing record details pages between interfaces.