The ability to lay things out on a page so they work well is essential to effective page design. Unlike making style judgements on printed pages, web designers need to work with differing sizes and types of monitors in mind.
Graphics design is also quite different on the web. Not all users are equipped with a digital artist's monitor and video card - they may not even have their graphics driver set above 256 colors. Another factor in designing web graphics is that color depth equals file size. To keep a page quick to load and keep the visitor's attention, use of the minimum presentable depth (and therefore size) of a graphic is necessary.