When you develop a website for a client you should usually be considering three or even four (if you also include staging) environments where one code base may be used. Local, development and live.
In this case you may want the code to behave slightly differently depending on where it’s being used.
A good example I regularly come across is when and where to send any emails that a system generates. When you are testing locally or on a development server you certainly don’t want to send out emails to people who have no knowledge of the system.