This webmaster tool is used to check whether a web site is banned by Google or not. Google bans web sites for various reasons and when banned, Google crawler doesn't visit the banned site anymore.
This results in the site not appearing in the Google search results and when that happens, there is no way the webmaster can hope to maintain the site.
Many small search engines too grab Google's search feed and the banned site gets carried to these search engines too.
To check Google banned sites...