Usage of canonical tags – on thin content pages and duplicate pages
A canonical tag (aka rel =”canonical”) is a way of telling search engines that a specific URL represents the master copy of a page. Using the canonical tag prevents problems caused by identical or “duplicate” content appearing on multiple URLs. Practically speaking, the canonical tag tells search engines which version of a URL you want to appear in search results.
via https://moz.com/learn/seo/canonicalization
Canonical tags are one of the important meta tags, but are very less used by website owners. When you add a basic SEO plugin like Yoast, it automatically adds the canonical tag to every post/page with the default URL that is same as the page.
You literally do not need to make any changes or settings for adding canonical tags.
You need to edit/modify canonical tags only under some special circumstances to improve the SEO
When there are multiple versions/copies of same page or some form of duplication, then you can refer the original link or the master page link as the canonical tag for all the posts/pages.
Find below an example where one page has 3 versions but only 1 is original or master and needs to be ranked in search engines.
1.yourwebsite.com/page-one/ – has default canonical tag as yourwebsite.com/page-one/
2. yourwebsite.com/page-one-1/ – a slightly modified version of “page-one” but it has its own canonical tag yourwebsite.com/page-one-1/ making it to appear to search engines as an unique page. Since it is not an unique page and it is a copy or slightly modified version of antoher page, you can change the canonical tag to indicate the master page, in this case yourwebsite.com/page-one/
You are telling search engines not to consider yourwebsite.com/page-one-1/ but to consider the master page yourwebsite.com/page-one/
This applies for multiple versions as well.
Adding proper Canonical tags helps avoid duplicate content and thin content issues. It also boosts the content value of the master page by redirecting some juice from the duplicate pages to it.
Note: Having multiple duplicate/slightly modified pages could help your visitor thats why it even exists, the canonical tag only explains this technically to the search engines.