Web design seque into SEO with some of the stats Google uses to evaluate a sites analytics. The average amount of time a visitor spent per visit on each of your pages, and how many sitewide links they click on, directly relates to your site ranking.
Sites with visually exciting but simple straight forward design, navigation and solid content, will hold a visitor or potential client/customer’s attention for much longer. If they are finding what they’re looking for, then they’re more likely to stick around and browse.
There’s a recipe to this, and if you ignore a single part of the algorithm, you’re just hindering yourself. You can have awesome content that is optimized and on point, but if your site design is outdated, slow, and confusing, then Google will subtract points, making fewer able to find your site.