What is the "Combine" option for?

[MrBlond]

On the edition page, under the line "Other editions", there are couple of options:
All editions / Add a new edition / Combine

Because I couldn't find explanation in the Librarian Manual, I ask here:
What is the Combine option for and how it works?

[lethe]

The combine function is for combining different editions of the same book. It is only available to librarians.

There is a section about combining and merging in the Manual.