Often bloggers keep multiple blogs, usually dedicated to different themes/niches, and sometimes they want to have a central "master" repository for all these blog posts, where the reader can find their articles without having to check each separate site. Or, you might be running a shared community blog, mirroring the posts that the different authors post on their personal blogs. Blog Mirror can help with all these.
Simply said, it copies every new entry in a remote site onto your blog, using newsfeeds. Whenever the remote site publishes a new entry, that sites RSS newsfeed will be updated with that content. Blog Mirror periodically scans all these RSS newsfeeds for new entries, and whenever it finds one, it copies it on to your main blog (from which Blog Mirror is running).