There are a lot of WYSIWYG (What You See Is What You Get) editors out there. They help beginners learn how to design web sites and shorten the time web developers put in designing web sites. They're easy to use and increase productivity. They write to the source code of the page the equivalent HTML code of the element the designer visually inserts (such as an image, table, horizontal rule etc). However, these editors often use either non-standard syntax or unnecessarily add to the source code.
Using standard HTML code means ensuring that the web site is written in 100% correct syntax. W3C, the World Wide Web Consortium has published specifications regarding HTML/XHTML and guidelines on how to use them to the best effect. W3C's involvement in this particular area of web development had primary as purpose the standardisation of correct HTML code usage.
For web developers eager to create standard compliant web sites, W3C has created a validation tool called the Validator. The Validator parses the source code of a html file and establishes whether it validates as HTML/XHTML document or not. The output reports any errors found during the process in order to help developers easily identify and fix them.
One of the reiterated recommendations of W3C is to always validate the web pages and correct them so as they become standard compliant. But why would people go to all that trouble? Parsing every web page, writing down changes, implementing them, parsing them again until 100% validation is obtained takes time and patience most of all.
Code validation does not help users as such but if done and if using standard HTML code, the web site can reach and be accessible to more people, users who use other browsers or platforms.