If you care about doing well in google organic with a site that has more than 20 pages it is absolutely necessary. Not using it is also a potential vulnerability where other people can link build to your pages using an alternative forms of the URL that still loads the same page content and convince Google that the page is widely duplicated on your site. Google picks which dupes go into the supplemental indexes and the one that stays in the primary index , if any, often isn't the one that rank bests. By using the canonical tag google knows if it has already indexed the page content under a different URL. This lets googlebot crawl large sites more efficiently and completely and it turns a dupe attack into a Page Rank booster.