Earlier quoted context omitted.
RSS likely isn't the only thing that uses it. XSLT is basically the client side declarative template language for XML/HTML that people always complain doesn't exist (e.g. letting you create your own tags or do includes with no server or build steps).
I understand that there are more possible uses for the tool, but RSS is the only one I saw someone mention. Are there more examples? It may be that I don't notice when I use it, if the page just translates itself into XHTML and I would never know until opening the developer tools (which I do often, fwiw: so many web forms are broken that I have a habit of opening F12, so I always still have my form entries in the net…
Practically every WordPress site with one of the top two SEO plugins (I'm not familiar with others) serves XML sitemaps with XSLT. It's used to make the XML contents human readable and to add a header explaining what it is.