301 redirects when implemented correctly can be a life saving effort when it comes to preserving the power of older web pages.
• You’ve moved your site to a new domain, and you want to make the transition as seamless as possible.
• People access your site through several different URLs. If, for example, your home page can be reached in multiple ways – for instance, http://example.com/home, http://home.example.com, or http://www.example.com – it’s a good idea to pick one of those URLs as your preferred (canonical) destination, and use 301 redirects to send traffic from the other URLs to your preferred URL. You can also use Webmaster Tools to set your preferred domain.
• You’re merging two websites and want to make sure that links to outdated URLs are redirected to the correct pages.