Define which URL parameters should be captured, and then creates a shortcode so that the value can be used in your Posts.
Why would I use this? So that you can pass info from the querystring to all links on your site. For example, perhaps you have an arrangement where you pay anyone who links to your blog a split of a commision you make on a sale. If they pass you a value on the querystring, for example refcode=affpartner1, you can then pass that into your affiliate links so that you can share commisions.