You need to do some tests at your website, but you don't want to break the frontend for the visitors.
A simple solution using Smarty.
Your current IP address is: 188.8.131.52
This way only you will see the template vars:
Another example, testing a module like CGFeedback without letting it show to the public.
This content is only visible at the frontend when you are logged in into the Admin, it is hidden for regular visitors.