Well formed HTML is easier for the crawlers to read. They are computer programs, after all, and expect HTML to be in a specific content. If the crawlers see no errors, your site is indexed more accurately.
Well-formed HTML is HTML that conforms to a specific HTML standard
The most common standards for HTML are HTML 4.01 Transitional and XHTML 1.0 Transitional
Verbose, duplicative, bloated, or error-prone HTML code makes it more difficult to get to the content, and decrease the content to code ratio