The second part to any successful website is programming. With the UX (user experience) completed, the UI (user interface) become the next crucial piece. Development includes any custom fuctions or features. The UI addresses what the website "does".
So what do you want your website to "do"? If you're looking for just a few pages of static marketing messages and a "contact us" form, then there probably won't be much programming required. However, if you want to manage custom content, allow users to login to portals to view private information, or some other interaction between the site and the user, then programming a successful UI is crucial.
Users must be able to manage and interact with content easily and without friction. Sites must react quickly and perform multiple functions simultaneously. Database calls, API services, and reporting views are just some of the functionalities required by complex sites.