Why use, Guest PM Notice?
When an guest views your forum, they will see a notification telling them they have a private message, and to view it you must fill out this form, When the user clicks on the word this, he is taken to your registration page, and hopefully will sign up, as it is human nature to be curious and they will want to know what the private message is.
Now this mod will only work if you have a Welcome Private Message set up on your site. If you don't have one you can set it up in ACP / Settings / Options / User Registration Options
So in theory, by adding this mod you may get guests to register, and hopefully become a active member on your site.