Information Architecture is rarely well thought out when a web site is constructed. Where a piece of content should live on a site and how your users, and search engines, are going to find it are important considerations so that the information flow is smooth and unrestricted.
A well thought out categorization schema not only makes the experience for your users more involving and enables them to find crucial information faster, but will help your site rank better in search engine results pages (serps) as the search engine robots will be better able to navigate and index all the pages that make up your site.
To get a better idea read SEOmoz's article on Information Architecture here...
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