why validate
There are good reasons to create valid html and css. Some of them are listed below.
For more detailed information see the W3C site.
For search engines
Search engines prefer valid html and css. Invalid html could be harder to parse for search engines and negatively affect your ranking.
See Google SEO Test - Google Prefers Valid HTML & CSS
For other browsers
Valid html and css are more likely to work in browsers that you haven't tested. That includes browsers that don't yet exist, like the next version of Firefox or IE.
For speed
Compliant browsers can render xhtml faster than html in quirks mode.
For accessibility
Screen reading software for visually impaired users works better with valid html.
For debugging
Invalid html or css could cause bugs that might not come up in other tests.
Valid feeds
Feeds are a simpler case: feed readers aren't as forgiving as browsers and probably won't read an invalid feed.