When the RF plugin is called in jenkins, it copies the Robot output files from the Workspace to the server.
By default those files are output.xml, log.html and report.html (this is customizable)
The plugin then uses the output.xml to build another XML file used to show the data about success/failures of the tests.
After that publishing step, output.xml file is not needed anymore (archived for possible traceability reasons though).
It would be a good improvement to make the archiving of output.xml optional, since in many cases it is not used and it might use a lot of disk space

This was discussed in Robot Framework User Group:
https://groups.google.com/forum/#!msg/robotframework-users/2pr6MmhmoKc/bgMJ6YWzUCYJ