fast-import/fast-export git/bzr: what is the importance of the marks files to history integrity?

I'm currently using git-bzr-ng to import a number of branches from a bzr repository and I've discovered that if you use different target branches for each imported branch the tool does not use the same import and export marks files.

Some reading of the git fast-import Documentation and reviewing of the bzr fast-import documentation examples suggests that the same mark's files should be used when importing multiple related but unmerged branches from the same repository.

Question is, how im开发者_运维技巧portant is this really? I haven't managed to find a good explanation of what these marks files are intended to protect from, so it's unclear whether if multiple branches have been imported without using them correctly there is a problem with correctly identifying ancestors, or if it's just for performance reasons to speed up subsequent branch imports/exports.

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John Meinel gave the following answer to the same question on the Bazaar mailing list:

bzr fast-import generates new revision information for every commit. If you specify a marks file, it will re-use the imports from the other branch. If you don't, the branches won't share common revisions.