When multiple developers work on a single website project it is obvious to use a source control software such as SVN, CVS etc. But you should do it even if you are the only developer working on the project.
Why? Source controlling helps an SEO expert to analyze the impact of a change on website traffic.
Imagine that you have modified some pages of your website (page title etc). After few days you wonder why the sales have dropped. You have totally forgotten the last changes you made to the website. Your source controlled website will help you to track every change made to it. You can easily recover back your sales.