Because the end result of all design is for the client, or viewer. As graphic designers, or designers in general, we need to keep in mind that the general public want things delivered to them as easy as possible. When designing for web you have to be conscious of the theme/concept, but also the way that users navigate, and how they are pushed through the website. Creating bite sized “loads” of information for people to digest is the best way for people to get through a project. Limiting the work of the user and the mental connections they have to make is what makes a successful webpage.