There are several situations where a particular category should be linked to directly. If the name of the category is known, but the ID of the category could differ (for instance, between production and development environments), then it is useful to be able to retrieve the category page link directly from the category name. In addition, it is helpful to not display the link at all if the category doesn't exist.
For this recipe, consider the situation where you need to link to three different categories: Testimonials, Portfolio, and Thoughts. You've established each of these categories in your local development environment and in your staging environment, but you haven't yet created them on the blog where you'll be launching your theme. This is a good situation to use conditional linking.
Given this situation, you need code similar to the following:
<?php
$nav_categories = array('Testimonials','Portfolio','Thoughts');
?...