User interface when it comes to a website is all about how a site looks, how it feels. How do journeys feel, how does functionality feel. Does in complement the rest of the site, the journeys and the intentions?
UI – is critical. Depending on the complexity of your site, your journey and your customers demands the requirement of UI can quickly increase. If you are looking at a site from the top down then a UI are critical to things like the styling and consistency and attention to detail that will make your customer convert.