But why would a server send me xml instead of html? From the thread you can see that the servers already serve html so why is there a second way to present that content via xslt?
For instance, I can provide an XML file that is a list of widgets, and then I can use to XSLT to give you a table with the values sorted in some way. I can't do that with plain HTML, I'd have to invoke Javascript or something.
It's similar to a modern web app talking to a REST API to get raw data in JSON form and then using that data in some user-friendly rendered web page. Except it's a standard that anyone can use, instead of everyone rolling their own client-side browser code.
Of course, XML is so complicated that it fell out of favour, but that was the idea.