I've been using the Drupal Category Module for a website. The owner of the site wants people to register for free on the site to view the content but to be able to still navigate through the site so you get an idea of what content is on the site first.
When creating a new Drupal module, I'd been trying to add a couple of links to a sidebar when viewing a user's details. I could of done this using a block but that would rely upon the block being setup to only appear on the user's page, and would also include all the markup that comes with a block. This is how I got around all that.