if you are having a site selling a product which may need use the same product page in different pages, this will create a duplicate content issue so to avoid this we can specify the canonical url (original url) to search engine by adding the rel="canonical" link to the <head> section of the non canonical version of the each page. for more info: http://support.google.com/webmasters/bin/answer.py?hl=en&answer=139394
Canonical Url tag is more important for e-commerce website because all e-commerce web store have huge number of product and those all are display in multiple url for one or more reason so in this case canonical tag is more useful. Google is one way to improving search result. Google panda and penguin update prove that Google strictly follow the rules against duplicate content.