You got the wireframes, brand identity and guidelines but of course, everyone is gonna visit your website and it has to look amazing. Seems like it's the most irrelevant step but so many still get it wrong and end up ruining the first impression the customers could have when they land on your website. Worst cases, it's so bad it overrides the great UX.

UI design is the color, layout, visual harmony of your website/app. the first 3 second user's take to make an impression is decided by the UI Design.

Utilizing the visuals to work for your brand is powerful.