This article (What a Canonical Link is and How to Use It Properly) says...

When a canonical link meta tag is included into a template, it essentially informs the major search engines that an entry page is part of a larger grouping of posts that should not be displayed in a duplicate manner within search results. For example, the tag could be displayed on every category and tag-based page that simply tells the search engine to consider those pages the same as a standalone entry page. It would then link together the standalone entry page, the category page, and the tag page, so that they are all perceived to be the same thing for the search engine. This boosts a website’s search engine optimization because the major search engines no longer consider every separate entry page to be a separate search result. They now view that content as a single, unified source, and they’ll rank it higher because it’s less “diluted” in search engine results. That can only lead to good things for the website’s administrator, as higher search engine rankings typically lead to more ad clicks, conversions, and e-commerce.