The less glamorous stuff first: planning the structure of a website. Some call it wire-framing, I consider it thinking-on-paper. Essentially it allows consideration of the underlying framework of a site before we get to the cool stuff - making it look good.
The initial planning makes the rollout of design concept fast, accurate to the brief's requirements, practical to use (UI) and on-brand. As well as accommodating the users experience (UX):