Graphic design sets the tone for a website. It creates the stage on which everything performs. If done well, it can enhance and highlight the words you say. If done poorly, it can distract and confuse.
Colors, lines and images on web pages evoke feelings in the readers. What kind of feeling do you want to create? Do you want people to feel calmed and comfortable? Do you want them to get agitated and move them to take action? Understanding about these things leads to a good graphic design than complements and supports your overall objectives.