These are pages other than your homepage, where the visitor has no idea how it fits in with the overall structure of your web site. You should always have a link to the homepage, so visitors know where they are on your web site.
Yes it is useful when search engine spidered back-door pages. You can not know which page will be get best rankings on the search engines. So, you should always have a link to the homepage on your all sub-pages.
