With fast moving competitive world continuous improvement is required to be in competition. These improvements can be reflected on the website. It can be anything like adding a new section, rebranding or moving onto a new Content Management System (CMS).
Need for re-design: For a client, it takes only 10 seconds to get impressed by the website and they can be visiting from different platforms available. If your website is outdated, Runs on flash and does not adapt the different mobile screen sizes, then it becomes mandatory for you to Re-design it. That is not only the reason for re-design if your Search Engine Ranking is low or visitors does not stick around for while you have to go for re-design. Lastly, the most important reason for redesigning your website is that it's taking more than 3 seconds to load.