bzr fast-import/fast-export what's the importance of marks file when dealing with multiple branches

[Bailey, Darragh]

While experimenting with using git-bzr-ng and import/export branches to 
git/from bzr I'm wondering what is the importance of using the same 
mark's files when handling related but unmerged branches from the same 
repository?

The example on 
http://doc.bazaar.canonical.com/plugins/en/fastimport-plugin.html 
indicates that one should do the following when exporting and import 
related branches:

bzr fast-export --export-marks=marks.bzr project.dev |
        GIT_DIR=project/.git git-fast-import --export-marks=marks.git

bzr fast-export --import-marks=marks.bzr -b other project.other |
        GIT_DIR=project/.git git-fast-import --import-marks=marks.git


But nowhere can I find an indication of why this is important? Is this 
just a performance improvement, or is there a repository history 
integrity issue should it not be followed?

-- 
Darragh

"Nothing is foolproof to a sufficiently talented fool" - Unknown