301 redirects are very useful if people access your site through several different URLs.
For instance, your page for a given ID can be accessed in multiple ways
It is a good idea to pick one of those URLs (you have decided to be
It is a good idea to pick one of those URLs (you have decided to be
http://yoursite.com/foo/ID/some-friendly-string) as your preferred URL and use 301 redirects to send traffic from the other URLs to your preferred one.