The .html extension is there because that's file type of the page. You don't see anything with the homepage because that's the default way that most servers will handle an index page. Visiting a directory at a URL, whether it's mysite.com/ or mysite.com/directory/, the server will display the default index.html file for that directory.
You can actually link to pages of your site without .html on the end and it will load the page, although it will append .html to it.
