It tells users (including search engines) where the next/previous/etc pages are in relation to the current page, giving them a structural context that might not be obvious (for a bot) based solely on regular hyperlinks.
In theory, it improves the ability of a search engine to return relevant information (e.g. if two results are otherwise ranked equal, but one preceeds the other, it probably makes sense to list the first as the primary match), and Google generally rewards the correct use of semantic metadata.
All of which hopefully means Google will pay more attention to the site in general, and give it better placing in search rankings.
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