The main reason for using canonical URLs is to prevent duplicate content on your website. Having duplicate content can be bad for WordPress SEO.
If search engines find two or more pages with the same content, then they won’t know which one to show in the search results. Because of this, all of these pages end up ranking lower.
Another issue caused by duplicate content is keyword cannibalization. This happens when multiple pages on the same website start competing for the same keyword, which can harm their rankings.
If a canonical URL isn’t set for duplicate pages, then Google will use its best judgment to choose a main page. The issue with this is that it may not be the page that you want search engines to index.
That’s why it’s important to set canonical URLs and tell search engines your preference. This way, Google will know which page to index and show in the search results.