Why XSLT

Note This page is redundant as WComponents is moving away from XSLT in version 2.0. WComponents does the XML to HTML transformation in Java and sends HTML 5 to the client. Alternate client rendering for modern applications is being built as I type...

Why XSLT?

Because Web Components is not ready.

In the future WComponents will replace XML+XSLT with Web Components and use JSON for data, but at the time of writing (2015) Web Components is not finalised nor is it widely implemented in major browsers.

XML+XSLT gives much the same benefits as Custom Elements. The server can generate "UI tags" without having to concern itself about implementation details or boilerplate HTML.