SEO oriented urls for Joomla!
URLs are what identify each of the pages on your website. Why do they matter so much? because search engines uses them in two very important ways:
As a unique label on which authority and other ranking factors will be attached
Have the same article or product page linked through several urls, and you will somehow "split" any authority/ranking value it may have. sh404SEF makes sure, from the start, that the same content is always accessed through the same url. Pages urls will not depend on your Joomla! site menu structure - unless you choose so, and will stay the same throughout the lifetime of your site. The following screen will list duplicates found for each page. They are all kept hidden from search engines and visitors, who only see one unique URL.
You can read more about patterns and SEO benefits of proper urls on this page from the MOZ.com site for instance.
As indicators of the structure of your content
The address of a website page can and should be used to give hints to search engines about how your content is organized and how various parts may relate to each other or to various topics. The default settings in sh404SEF will generate short but structured urls: articles pages will have urls such as /joomla-seo-analytics-security/sh404sef. This sample url structure will give a tiny indication to a search engine that the software described on that page, sh404SEF, maybe relevant to concepts and domains like Joomla!, SEO, web sites analytics and security.
So sh404SEF will construct URLs that benefit your web site, but it will also build a database of all the pages on your site. You can then access any url on your site, with a form such as the one below:
You can manually customize any URL. It can sometimes be useful, for instance to mimic pre-existing URLs from older web sites.
Opposite to these good practices, a common mistake is to "over-optimize" URLs and try to insert as many keywords as possible in hope to improve SEO and visits. This first won't help at all, and second may only have negative effects. Search engines may see this as attempts to game them (and retaliate!). At best, it'll just confuse them. Though you can manually customize each and every URL on your site through sh404SEF URL manager, do so only with great care!
Another page from MOZ.com states very clearly, under the URL section, why it matters to help search engines understand all about your content in every possible way.