Although browsers are indeed important users of HTML and
XHTML, there are other programs and systems that read those
documents. Search engines for instance read documents, but
are not browsers. By using the term "user agent" we are
trying to remind people of the difference.

For example, when you do a Google search often you will see
under some of the search results something like "This web
page uses frames, but your browser doesn't support them."
therefore surely frightening off some people from clicking
on that link. The author of the website in question hasn't
realised that there are more than just browsers, and that
they ought to include better text in their <noframes>
section, so that they don't appear so foolish when people
search their site.