What are the effects of the "access commerce administration pages" permission
Does it do nothing alone and only work if other permissions are added?

Currently, yes (although that could theoretically change in the future).
The permission is added to all HTML admin routes for the order entity type (add/edit/etc), and for admin overview pages, which essentially list sub-pages that the user has access to.
If the user doesn't have access to those sub-pages, all provided by separate permissions, they won't see the links, and won't be able to visit them. Similarly, if they don't have the correct entity access permission, they won't be able to perform the respective operation on the entity.