Confidence may be misplaced.

I'm confident of writing valid code: I know the general rules. But sometimes I make errors. Constant background validation alerts me when I make a mistake.

I just discovered that my javascript, which was generating extra (presentational) markup, was making my HTML invalid. I did not detect this, because the W3C validator checks the page without javascript transformations (and so does my background validator).

My error was to put <div>s inside a <dl>. They are not allowed there.

I suspect this mistake was the cause of an irregularly occurring, unpredictable browser rendering bug. Validation greatly reduces the need for bug-hunting.