Changing of URLs causes the following problems:
People who bookmarked the page can't find it again
Search engines can't find your content
People who link to your pages now have broken links
The best advice, then, is to never change your URLs. The practicality, however, is that there are many reasons that a URL might need to be changed:
Keywords change so content that was relevant before doesn't fit
Redirects came into being because webmasters need a way to tell visitors and search engines that content has moved from point A to point B on the server. In its most basic form, a redirect simply tells a browser to go to a different page. For example, you want people who visit www.yourDrupalsite.com/expensive-cars/Ferrari to automatically move on to www.yourDrupalsite.com/cars/expensive/Ferrari. This helps to maintain a clean site that doesn't break...
