topbanana already provided the key point in his response—visibility of a great deal of context. It's especially useful for programming, allowing for, for example, related code side-by-side. A large desktop also allows some user interface paradigms such as drag-and-drop to work more elegantly. Drag-and-drop on a small monitor is often frustrated by the need to first arrange the source and destination so that both are visible at the same time. Generally speaking, a large amount of desktop space allows you to spend less time fussing with windows to get at what you need and more time just working.