GUIs swing back and forth between 3D and flat ever since Microsoft released Internet Explorer 4 for Windows 95 which introduced flat GUI toolbars. That watercolor Office release had the look planned for WinXP but Apple released Mac OS X with its (for that time) super realistic 3D icons and GUI widgets and Microsoft wanted to counter that. In fact, most such changes were initiated by Apple and were never about functionality.
GUIs swing back and forth between 3D and flat ever since Microsoft released Internet Explorer 4 for Windows 95 which introduced flat GUI toolbars. That watercolor Office release had the look planned for WinXP but Apple released Mac OS X with its (for that time) super realistic 3D icons and GUI widgets and Microsoft wanted to counter that. In fact, most such changes were initiated by Apple and were never about functionality.