Quick answer: Canonical tags affect SEO from two points of view. For once, they directly influence how search results display. They can also influence the general rankings of a website due to multiple factors, such as structure, user experience and PageRank flow.

Is canonical tag important for SEO?

A canonical tag, also known as a canonical link or "rel canonical," is a tag in the source code of a page that indicates to search engines that a master copy of the page exists. Canonical tags are used in SEO to help search engines index the correct URL and avoid duplicate content.

Is Canonical bad for SEO?

Be careful canonicalizing near-duplicates

There's a lot of debate on this topic, but It's generally ok to use canonical tags for very similar pages, such as a product page that only differs by currency, location, or some small product attribute.