When advanced features are enabled in Active Directory Users and Computer (ADUC) you will have access to additional functionality in the snap-in. Two of the most useful functions available in advanced mode are the Object and Attribute Editor tabs. Further, the AD structure is hidden in basic mode. One of your most important containers, System, can only be seen when Advanced Features are turned on.
*The attribute editor is available in Windows Server 2008 R2 and above.