These pictures often include smaller images, pieces of typography, and now moving images and sounds. Web pages are advertisements. The nature of the web is that people surf casually, and if you want them to read your material you need to get them to stop at your page. To do that, you need to get their attention, and promise them a rewarding experience. First impressions matter.
Magazines work hard to make attractive covers, and web page designers do the same.
Even if a page is simply reference material, if it's poorly organized or difficult to read, people will not enjoy the experience, and will either not return or regret it when they must. So good design is essential. Web design combines traditional graphic design with the limitations of screen display and the active capabilities of web browsers. With today's markup tools, almost anybody who has absorbed a sense of visual aesthetics from the culture can create an acceptable set of pages. But to make pages that are interesting, or exciting, or beautiful, or intriguing, or otherwise special, some extra effort is required.