Graphic design creates visual logic and seeks an optimal balance between visual sensation and graphic information. Without the visual impact of shape, color, and contrast, pages are graphically uninteresting and will not motivate the viewer.
Visual and functional continuity in your Web site organization, graphic design, and typography are essential to convince your audience that your Web site offers them timely, accurate, and useful information. A careful, systematic approach to page design can simplify navigation, reduce user errors, and make it easier for readers to take advantage of the information and features of the site. (Lynch & Horton 2001)
There is quite a difference in opinion in how usability and graphic design are intertwined on the web, as discussed in Usability Experts are from Mars, Graphic Designers are from Venus and Usability is not Graphic Design. The web site of well known usability expert Jacob Nielsen is described by the University of Greenwich as "inelegant".
Some of the ways graphic design support usability, a walk through arranging information and analyzing graphic design.
Some interesting comments by Anne Harris in Building for Usability. She suggests instead of creating a plain vanilla looking site by building down to the lowest common denominator, we aim high and build in the ability to scale down. Makes sense.