XSLT is really separating (XML) data from markup in the case of the web. More generally it's transforming between different XML formats.

But in the case of docs (eg XML-FO for docbook, DITA etc) XSLT does actually separate content from styling.

Yes that's why XSLT is such a natural fit when you learn about HTML+CSS. It's the same idea, but applied to HTML templates, which is something you immediately want when you hand-write HTML (e.g. navbars, headers, and footers that you can include on every page).