[ Thanks to Anonymous for this link.
]
“When you are updating website you need to send error 503 to
client. Error 503 indicates that Apache or Lighttpd web server is
temporary out of service. This is useful if you are running popular
database driven community or e-commerce website. Error 503 informs
search engine that site is temporary out of service. This is quite
important for site which is heavily depends upon search engine for
selling products and services. So when the site is under
maintenance you can send user a good message indicating that site
is down for some work.”