Here are a list of statements from Google about the Canonical tag:
Canonical links let you explicitly specify which version to crawl. There are multiple advantages to this:
- You get to specify which URL should appear in search results.
- It helps search engines consolidate multiple URLs into a single, preferred URL. For example, if other sites put query parameters on the ends of links to your page, search engines consolidate those URLs to your preferred version.
- It simplifies tracking methods. Tracking one URL is easier than tracking many.
- It improves the page ranking of syndicated content by consolidating the syndicated links to your original content back to your preferred URL.
- It optimizes crawling time. Time spent crawling duplicate pages is time not spent crawling other pages with truly unique content.
As you may know, Canonical tags helps crawlers to understand which URL is the right one if multiple page URLs exist with the identical contents. It will also speed up the crawling process when several URLs exist
The most important statement is in bold. A canonical tag prevents google juice leakage between several URLs. It will help to consolidate the PageRank to one preferred URL. In other words, a page can benefit from a canonical tag when other URLs exist for the page content and when they aren’t linked by a 301 redirect. It also means that with a canonical tag in place, the page will get higher in the google search results.
This confirms why it is so important to add a canonical tag to every post, page and product page.