A canonical URL allows the Site owner to tell search engines about the designation of any given page. This mechanism serves a number of purposes, allowing;
- One or more pages to designate another page as authoritative,
- Content to evolve over time by producing new fresher content and pointing the old pages to the new one
- Pages to coexist (in the case of products) without the Site being penalised for duplicate content.
While Google clearly states it is not obliged to use the canonical if it feels another page is a better choice than the nominated one, it tends to index pages more rapidly if the page contains a [self-referencing] canonical URL.