Imagine going around a database or your folders on your hard drive in a text based only environment. This is a task normally done by the technical user. A GUI allows you to use pictures and icons (or other images) in order to make sense of what is going on. Some programs or Operating Systems may use text that wouldn't make sense to the average/basic user so using images to represent the task it is you are doing makes things a whole lot simpler. Not only that but using a GUI allows you to make a organized environment that a user can be familiar with. its kind of like a standard that isn't necessarily a rule.