Not much I guess...it just looks more professional I suppose. I make sure all my sites validate, it really has no effect on anything though.
Validation enhances interoperability -- correct rendering in all user agents. However, web pages can, and do, fail validation and still display pretty much as expected in graphical Browsers anyway due to their built-in Markup discrepancy compensation -- in other words depending on the error compensating properties of individual Browsers. Validation does catch many easily corrected Markup errors and the resultant code is consequently easy to maintain or change. Pages containing invalid Markup may not display or function correctly in Screen Readers, BRAILLE interpreters and Textual Browsers or when incorporated into other applications. As we move toward the Semantic Web, the rigors of XML will result in a greater requirement for Valid Markup. Valid and well formed Markup also bespeaks careful craftsmanship and that appeals to many web authors. James