In most cases, websites are designed with the home page as the starting point, guiding users down through their structures, and providing them with ever more detail about the topic being discussed. Often, when a user arrives at the page they were looking for, that page will contains some very specific information.

For example, someone entering a car company's website might start at the home page, and will be made aware that cars are the topic being discussed. Therefore, by the time they pick a model, select "Interior", and navigate down to the "Leather Options" page, it's no longer necessary for the site's copywriter to mention that cars, or even car seats, are being discussed. It's pretty much obvious.

Dropping-in on the conversation

However, for search engines and their users, any page on the site can be an entry point. Search engines prefer to send users to pages that have specific information, because their job is to find an exact match for the words the users is searching for.

Search engine users, arriving at a site's internal pages, won't have the benefit of the context established by those upper-level pages. They walk into a conversation that already started, and have a harder time figuring out what's being talked about.

Therefore, both for the sake of increased search engine rankings and increased usability, webpages should have content that stands on its own merits. It should be a complete self-contained unit of information, even as it performs its role as part of a greater website.