HTML and other Internet related standards are evolving so fast, it's becoming harder to keep up with the latest. You may want to validate your HTML pages from time to time, just to make sure that you're not off the charts as far as the latest HTML standards goes.
Yes it's true that newer HTML standards are backward compatible with the older versions for the most part; but since some tags are being depreciated in favor of newer tags, it's a good idea to check your tags every now and then. Using valid HTML will not only make sure that your pages will continue to be portable, but it'll make it easier for those who develop HTML software to write more reliable tools.
Another reason to validate your web pages is to make sure that there aren't any typos or other mistakes, especially if you write all or portions of the HTML code manually. A single unnoticed mistake may prevent your page from properly appearing on different browsers or getting a better rating in search engines, for example. Some validation services will even check for spelling mistakes and other improvements.