Today, you can share a file only with multiple communities if you store it in your personal files. Besides other limitations of this approach (e.g. you cannot add a shared file into a community folder), the major problem is that this file is lost if the user leaves the company and gets inactivated/deleted.
You could add a copy of a file to each community, which makes it hard to keep it updated in all places.
Request by customer is to also allow sharing of community files with other communities. This could reduce information silos and storage footprint as the file is not going to be uploaded multiple times. Sharing should be even possible from a restricted community with other public communities. In some cases a file in a restricted community can be publicly available and shared with others.
"Share by link" does not fully cover this use case:
1. It is not visible who the file is shared with.
2. The "shared" file is not listed in the other community library.
3. Sharing is discontinued as soon as the share link is removed.