What is a canonical tag?

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.

Why does canonicalization matter?

Copy content is a muddled subject, yet when web indexes creep numerous URLs with indistinguishable (or basically the same) content, it can cause various SEO issues. To begin with, assuming pursuit crawlers need to swim through an excessive amount of copy content, they might miss a portion of your one of a kind substance. Second, huge scope duplication might weaken your positioning capacity. At last, regardless of whether your substance rank, web indexes might pick some unacceptable URL as the "first." Using canonicalization assists you with controlling your copy content.

Reasons to choose a canonical URL?

There are a number of reasons why you would want to explicitly choose a canonical page in a set of duplicate or similar pages:

1.To specify which URL that you want people to see in search results. 

You might prefer people reach your green dresses product page via :

https://www.example.com/dresses/green/greendress.html 

rather than 

https://example.com/dresses/cocktail?gclid=ABCD

2.To consolidate link signals for similar or duplicate pages.

It helps search engines to be able to consolidate the information they have for the individual URLs (such as links to them) into a single, preferred URL. This means that links from other sites to:

http://example.com/dresses/cocktail?gclid=ABCD 

get consolidated with links to 

https://www.example.com/dresses/green/greendress.html

3.To simplify tracking metrics for a single product or topic

With a variety of URLs, it's more challenging to get consolidated metrics for a specific piece of content.

4.To manage syndicated content.

If you syndicate your content for publication on other domains, you want to ensure that your preferred URL appears in search results.

5.To avoid spending crawling time on duplicate pages. 

You want Googlebot to get the most out of your site, so it's better for it to spend time crawling new (or updated) pages on your site, rather than crawling the desktop and mobile versions of the same pages.