Well-formed web site, why is it important?
Search engines scan web sites found on the internet (I'm saying that because this work also has an intranet) with some programs, they gave the name of the robot and take their databases.
During the scan, smart robots will index texts which are on your site smoother and faster by taking advantage of the markup language standards. Bad formatted standards thanks to a website that was created outside the smart robots can be indexed, though the robot's job could be the next indexing time make it difficult to later.
In short, the search engine's web site in order to more properly categorized as faster scanning and it is important that a proper formatting.