Why would you need a "Valid XHTML & CSS" notice at the end of a page
css, html, standards-compliance, user-interface, w3c-validation
Solution
I can think of a few possible reasons:
It may be a marketing tool. "Look, we code to appropriate standards!" This could apply to the individual who designed the site (and might include it in their portfolio), even if the company as a whole is non-technical.
It could be a means of spreading awareness about XHTML, et al. "You should code to appropriate standards, too!"
It might improve the perception of quality for the site as a whole (and maybe the company's products, by association). I don't really think this is particularly likely, but some marketing departments might.
As others have noted, it could just be a point of pride for the web designer.
Problem
I've seen this in the footer of various websites, most of them non-technical websites. Some websites go even further and include a W3C badge stating the fact. I don't see how this can be of any help to the targeted audience.