If you have imported your blog from blogger (blogspot.com) then you might be redirecting visitors from your old blogspot blog to your new wordpress blogs HOMEPAGE.
While this approach ensures maximum hits it may not convert people coming from your old blog to loyal visitors. As those people may be referred to old blogspot blog by someone else. If they came looking for something specific then they may fill lost!
So this plugin just takes care of this part. It checks for which post people were looking on old blog and then redirect them to same post but on new blog!
For more details....