The canonical URL is a Meta tag that allows the search engines to index the preferred URL for one site from the multiple URLs available. The process of Canonicalization is the process of selecting the canonical URL for one particular site.
When searching for content using a search engine, you will discover that there is more than one URL that will lead to the same website; however only one of these will be the canonical URL for the site. When web masters add a canonical URL for each page they can avoid any potential duplicate content penalties from the search engines, as well as ensuring that users land on the correct page every time.
In 2016 Google added the rel=”canonical” tag to make it a simple task for web masters to let search engines know which is the preferred version of each page on their site. Prior to this it was necessary to redirect every URL to the preferred URL, which could be time consuming.
Using a canonical is especially important for ecommerce stores, where a single product might have a dozen different pages based on how it is accessed by users. Without a canonical page, all of the links going to these pages are split rather than being consolidated, and there is a possibility you could suffer from a duplicate content penalty.
While it is true that the search engines will attempt to determine which page out of several variations is supposed to be the canonical page, they don’t always get it right. In SEO terms, it is always better to have a canonical reference versus not having a canonical reference.