This plugin allows users to authenticate using a password in the URL. This is especially useful to allow feed readers to access private feeds.
Some (half-baked) feed readers do not support private blogs. They will fail to retrieve content if you protect your site with either the regular WordPress signon or HTTP authentication. As a work-around, it's useful to be able to create a URL that contains the login information. That way, even the most basic feed reader will be able to show the private posts you have access to. (But see the warnings below).