The DOM is an API for accessing, updating, creating, and deleting the elements, properties, and content of XML-based documents. This includes documents in related, but not strict XML, grammars such as the latest HTML specification.
For the average developer, doing a ton of pure DOM manipulation is pretty rare these days. jQuery took care of that many years ago and it's never come back into fashion. I can say from experience that it's still useful to know how DOM manipulation works under the hood so that you can code yourself out of a bind when you run into something the library or framework you're using doesn't provide.
It also illustrates the possibilities of what's available when working with different technologies. It's one thing to have access to something that a library or framework author finds interesting, but...
