Things like entire information applications.
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Everything you see is XQuery running on MarkLogic Server (along with the source to prove it).
This is why XQuery is more than the SQL for XML.
Information products are about content. So, in the same way you could imagine a 'data' application being written in SQL (returning just tables of data), XQuery allows you to work with content and return a native content application.
How? First, information products delivered to the browser are XHTML - an XML standard which can be an output of XQuery.
Second, XQuery is a powerful development language. It has a rich API that lets you operated on XML, strings and numbers.
And the best part is that information products built with XQuery work natively on the content source -> XML.
There are no intermediate steps and confusion of turning XML into data then into objects then into text then into HTML.
XQuery lets you work directly against the content in its native environment.