What does the "keep" flag really do?

In the documentation, it is stated that the keep flag allows to keep deleted files and directories during incremental backups.
I’ve tried to do some incremental backups with and without setting this flag after having deleted files, but did not see any differences.
Can someone explain me what exactly is the purpose of this flag ?

Sorry to bring this topic back up, but I’m still interested to have an answer about this flag. :slightly_smiling_face:

Really, no one?
Maybe @uroni could enlighten me on this?