For an organization that does not have Windows Active Directory already in place, that is
one place to start because Active Directory Domain Services is key to application and user authentication. For organizations that already have a fully operational Active Directory running on Windows 2003 or Windows 2008, upgrading to Active Directory Domain Services on Windows Server 2008 R2 might be something that is addressed a little later in the upgrade cycle when AD DS 2008 R2 functionality is needed. To get a lot of the Windows Server 2008 R2 server functionality like 2008 R2 DFS, SharePoint Services, Hyper-V virtualization, and so on, an organization can still run on an older Active Directory environment (typically Active Directory 2003 native mode). However, the point is that Active Directory 2008 R2 is not a prerequisite to get Windows Server 2008 R2 server role functionality.
Because Active Directory is more than a simple list of users and passwords for authentication into a network, but rather a directory that Microsoft has embedded into the policy based security, remote access security, and certificate-based security enhancements in Windows Server 2008 R2, AD DS 2008 implementation does occur earlier in the migration cycle for organizations wanting to implement many of the new Active Directory 2008 R2 technologies, such as Active Directory Recycle Bin, Offline Domain Join, Managed Service Accounts, and the ability to use PowerShell cmdlets within a Group Policy Object.
Windows Server 2008 R2 extends the capabilities of the Active Directory by creating better management tools, provides for more robust directory replication across a global enterprise, and allows for better scalability and redundancy to improve directory operations. Windows Server 2008 R2 effectively adds in more reliability, faster performance, and better management tools to a system that can be leveraged as a true enterprise directory provisioning, resource tracking, and resource management tool. Because of the importance of Active Directory to the Windows Server 2008 R2 operating system, plus the breadth of capabilities that Active Directory can facilitate.
Source of Information : Sams - Windows Server 2008 R2 Unleashed