Website redesigns, when done well, improve the experience of visitors through better navigation and visuals. In turn potential customers will see your website more often in search engine results, visit more often, stay longer, and seek your services. 

Website design is not a static profession, it is creative and technical, meaning that your competitors will serve up a better experience with their own website redesign. After enough time has passed, a website redesign is not an option, but a competitive necessity.