The 80-20 rule is one that applies to several areas of business. This includes SEO. There are some elements in SEO that take a large amount of time and effort, especially in areas like building backlinks. In this case, deciding on which 20% of these can provide 80% of your SEO value can save lots of time while not cutting many results.
In addition to this, when you have hundreds (or thousands) of pages on your site, it’s important to try avoiding the duplicate content penalty where possible. To do this, each page must be unique and have valid content. With that many pages, even with a full staff it still takes a lot of time. In this case the 80-20 rule can be applied to choose the most important of your site’s pages and make sure they’re prepped to be crawled fully by the search engines. You can find more detail on how to apply the 80-20 rule to SEO from Eric Enge at Search Engine Watch.