You can import and export subtrees and individual nodes in the web tree, for backup purpose, or to transfer and re-use in different websites or projects.
The use of these functions is rather self-explanatory.
Use Export XML if you simply want an XML representation of the exported node, for import into the same web, or processing of the XML. Associated files (images, pdfs, etc) will not be attached to the XML export.
Use Export As File... to download a zip archive of the node XML, and associated files. This is equivalent to a full backup of the node. You can later import this file to restore the node to the backed up state.