Clearly it's important to design your pages properly. They all have to be able to stand on their own when people are viewing them, but there's more to a website then technically functional pages.
A website is much more that a collection of web pages. A proper web site is a complete system, designed for a specific purpose. The focus here is on the "complete system" concept. Modern search engines don't rank web pages in a vacuum. They consider a large number of variables involving both pages that link to a given document and the web pages that the document links to. In most cases, these variables are somewhat out of your control. You have limited control over who links to you and though you have control over which pages you link to, you have no editorial control over their content.
You do have control over the other pages on your site. You have complete control over the links and associated link text on those pages as well as complete editorial control of the surrounding content. This provides you with the opportunity to leverage this control. Though using this resource can seem intimidating if you look at it on a page by page basis, leveraging this resource becomes simpler the broader you think. Think of your website not as a collection of individual pages, but as a complete system.