Why We demote URL through Webmaster?
 

My question is Why We use demote to block URLs in Webmaster?

If you think that a sitelink URL is inappropriate or incorrect, you can demote it. Demoting a URL for a sitelink tells Google that you don't consider this URL a good sitelink candidate for a specific page on your site. Google doesn't guarantee that demoted URLs will never appear as a sitelink, but we do consider a demotion a strong hint that we'll try to honor when generating sitelinks.