For SEO, each page should only be accessible through one specific URL. This is your canonical URL.
Google discovers content on your website by crawling it. That means: it follows the links within your website (and from outside your website) and evaluates content based upon the URL it discovers.
While discovering content, it might find that the same content is reachable through a different URL. If this happens, Google has to decide which URL is the official/proper/intended canonical URL. Sometimes Google chooses wrong. That’s why we need to make sure we have one canonical version of each URL upon which we’re focusing.
Once we’ve decided which is the canonical version of a URL, we can tell Google which URL we think should be canonical. This prevents Google from wasting its time on our site and improves the chances Google will serve the correct URL to our visitors.
Even if you’re not doing an SEO campaign, having a canonical version of each URL will make your life a lot easier.
Having a canonical URL will help your SEO efforts because: