WP enqueue style correctly?

Ok, so I am working on my first wordpress theme and looking at other peoples code for help and I came across something which really confused me in the header.php. My question is why do you deregister the style.css before registering it? is this best practice or something.

<?php
wp_deregister_style( 'style-css' ); /* Why? */
wp_register_style( 'style-css', get_stylesheet_uri() );
wp_enqueue_style( 'style-css' );
?>