It seems to me that validation would be especially helpful when you're composing one Web page out of a bunch of smaller components; if, say, each component is in its own <div> and your validator confirms that it's a proper <div>, then you can have more assurance that your code will stitch them all together into proper <html>. If one of the components is invalid, you run a greater risk that when that component is included in a page with other stuff, the interaction among all the pieces will create an ugly surprise.
