Why do people redesign their websites? Reasons are abundant, and here is a list of the most common ones.
Modern look: You don't want you site to look outdated, so from time to time you brush it up in line with the latest trends.
More trust in the brand: If site owners continuously invest into making their sites better, users tend to associate trust their brands more.
Growing needs: Your business evolves. You might be changing the focus of your activities, your message, and your marketing methods, or you might even be repurposing your business—and so you need a redesign to make your site match those needs.
Higher conversion rate: Your current conversion rate is low and you think you can improve it if you change your sales copy's layout, create some extra pages that describe your products, revise your return policy, and make other tweaks.
Increased user engagement: You can improve user engagement in various ways, and you need a redesign in the majority of cases. You might feel that you can improve user engagement if you add videos, visuals, user reviews, so a redesign is one of the steps you take.
Better SEO and navigation: You might be moving to a different CMS that has built-in SEO capabilities. Or you're simplifying the structure of your site to streamline indexation. Or you're adding a blog to have an opportunity to add more fresh content. One way or another, you're redesigning the site for better SEO.
Taking into account my area of expertise, I'm obviously most interested in that final point: redesigning for better SEO. Although many people turn to redesign to boost their search engine optimization, in reality a redesign might have a negative impact on one's previous SEO achievements.