The idea of a complete system is rooted in the concept of theming. In a nutshell, theming is the organization of your linking structure in a hierarchical tree-like manner, starting with a broad topic and working your way towards more specific topics as you get closer to the leaves of the tree. Brett Tabke has put together a great diagram that demonstrates the concept of theming more clearly. Some will argue that theming isn't used by search engines, but that's still up for debate. Even if it isn't, following the concept will make it easier for you to recognize which pages make sense to link together, which will help you out in either case.
What this all means in a practical sense, is that whenever you want to help your rankings for a particular topic, it's helpful to use your own site's existing resources as a first step. If you have other pages on that topic, see if you can find ways to link them appropriately. If you don't have any pages on that topic, consider writing some. Aside from providing some supporting infrastructure for the page you're trying to get to rank well, you'll have new pages which may rank well themselves.
Let's also not forget a major side effect of this type of site development. By interlinking pages that cover similar material and expanding existing sections you're trying to rank well for, you are also creating a friendlier environment for your website's audience.
Your audience will be able to find on-topic material quickly and easily on your site. Though search engines have become very good at producing relevant results to particular query, people like to find all that they need in one place. You should always do your best to make it easy for them to do so.
To review, it's important to design your pages properly. Without properly designed web pages, you won't be able to deliver your message. But it's equally important to look past the page level and see your site as a complete system. Your website is more than just a collection of web pages. Once you see your website in this light, you'll have a much better chance to rank well for keywords of interest and you'll provide a better overall experience for your visitors.