XML is useful because it means that your content can be used
in different situations easily.
Once your topics are in XML (XHTML) you can then easily
transform them to a webpage output, generate high quality pdf
output.
I have a few posts about this:
http://techwritetips.wordpress.com/tag/what-is/
At the moment, one of the main reaons I largely use Flare
rather than RoboHelp, although I have both licenced is the
flexibility that XML content gives me for doing things like high
definition pdf's:
http://techwritetips.wordpress.com/2006/07/19/best-pratice-flare-3-high-definition-pdf-part-1/
I could export from RoboHelp to xml and then perform these
actions, but this introduces an additional step. Something that HAS
to be in the next RoboHelp is xml topic content (something like
XHTML), then it will be a real force again.