Because you do not want to create web pages, but to render some information in the form of web pages. And as you write that information you make distinctions unique to a) this information and b) your approach to it. And one of the best ways to do this is to come up with a custom set of XML tags. You write about chess? Fine: invent tags to decribe parties, positions and moves. Or maybe a tutorial on Esperanto? Fine; invent a notation to highlight the lexical structure and the grammar. You can be as detailed as you want and at the same time you can ignore anything you do not care about.

And then you want to merely render this semantically rich document into HTML. This is where XSLT comes in.