why we set base url in codeigniter code example

Example 1: dynamic base url codeigniter

defined('BASE_URL') OR define('BASE_URL', (is_https() ? 'https://' : 'http://') . $_SERVER['HTTP_HOST'] . dirname($_SERVER['SCRIPT_NAME']) . '/');
$config['base_url'] = BASE_URL;

Example 2: fix working base_url for local and server both in codeigniter

$config['base_url'] = "http://".$_SERVER['SERVER_NAME']."/project_name/";
this way you config you base_url , then won't worry about in hosting. both works in localhost and server.