Icons, themes, windows, visual control components and etc that made up the GUI represent the outer identity of an OS. Power and serious users, programmers and geeks favor the command prompt /shell to reach the inner working of an OS, but GUI appeal to most people: they are mimic objects that users are so everyday accustomed to, and icons, linking commands being actually “files” (note again a metaphor) buried deep down the sea of paths and directories, are a memory aid, serving greatly to reduce the chance of mistakes, and with their appearance foster an environment of affinity.
The BeOS/Haiku icons are an integral part of the OS, they should serve, progress and shine as the underlying codes.