Some people think that when they have a website they can leave it for months and even years as it was when it was first built. The search engines like to see websites that are offering the most relevant and up to date information, so will favour sites about yours that are constantly adding new content to it.
Updating your website is not only for search engine optimisation purpose but also for the way it functions and the way it looks. As with anything, websites can become outdated it nor changed often and you may be missing out on the experience that you can offer to your customer by not adding in new functionality such as online calculators, live chat facility etc. if you do not do any updates.
Simply adding a case studies or recent projects section to your site is a great way to get that all important, relevant content on the site without cluttering up the main navigation and pages.