Website Maintenance: Importance of HTTP 503

Include below code for maintenance of site:

define('OT_MAINTENANCE',false);
if(OT_MAINTENANCE)
{
   header("HTTP/1.1 503 Service Temporarily Unavailable");
   header("Status: 503 Service unavailable");
   header("Retry-After: 3600");   
   echo '<h2>Service not available. Site under Maintenance. Please try later.</h2>';
   exit;   
}