What's with stylesheets in the first place?

XML is not a fixed tag set (like HTML)

XML by itself has no (application) semantics

A generic XML processor has no idea what is "meant" by the XML

XML markup does not (usually) include formatting information

The information in an XML document may not be in the form in which it is desired to present it

Therefore there must be something in addition to the XML document that provides information on how to present or otherwise process the XML