The default Drupal 7 comment permalink has the following format,
/comment/[comment-id]#[comment-id], if a node has 10 comments, each page with the URL
/comment/[comment-id] loads identical content to the node itself.
10 comments is just fine but how about 1000 comments? 1000 duplicate contents is very bad for SEO and our website may be the target of Google's sandbox.
So this module rewrite the comment URL format to
/[node-alias]#[comment-id] and than redirect the old URL format to new one.
Note: this module only rewrite the comment permalink generated by core comment module, comments generated, let say by Views module not changed but redirected to proper URLs.