What good is following the W3C HTML/XHTML Specification?
Interworking specifications give us an environment where we can expect consistency. They reduce the need for rewrites and fixes (although lack of specifications support in common browsers dilutes that benefit) and they allow us to publish materials that have a greater chance of succeeding in a wider context. The WWW wouldn't have been very successful if it had been designed to run only on NeXT.
It is much safer to follow a known reference when writing HTML pages than the moving target represented by new versions of Internet Explorer and Netscape Navigator. An adherence to the available HTML recommendation ensures that the author won't have to constantly update his pages each and every time a new compliant browser enters the market.
We as web authors are dependant on browsers to get our information to the visitor. The browser manufacturers are dependant on us as a source of information for its users. There is a symbiotic relationship between us and the browser manufacturers. One good way of making sure one party doesn't dominate the other is to avoid proprietory markup. By following a documented recommendation, such as the HTML or XHTML specification, we have a goalpost that both the web author and browser manufacturer can aim for.
Building a valid page is a starting point. A basic how-to for building web pages using HTML, written by a forward thinking organisation (World Wide Web Consortium) as a recommendation which will still be viable when the next big step comes along (like the Semantic Web).
The home page of a daily newspaper might not last very long, and it might not need to reach out to users of search engines, so doesn't have to follow a forward thinking standard. But imagine a potential client who wants to build a site that's successful in itself, with or without a paid marketing budget. He needs to hire a person or company capable of working in a WWW friendly manner, who can make a page that'll survive the next generation of browsers, and that works in the non-visual context of search robot spidering. That's an example where following a documented W3C specification is an advantage.
Its a bit like driving on the wrong side of the road. Yes, its fun and cutting edge, but since everyone else is concerned about getting to their destination safely, they stick to the right side of the road. Driving on the wrong side of the road can result in a big mess if you are not super careful. Following a standard allows people to work together easier to aim their common or personal goals.