The Admin Guide Chapter 17 has a summary which is the motivation for using 
Active Directory to publish TSM information.
Basically this allows the specification of COMMETHOD ACTIVEDIRECTORY in 
TSM Windows client client option file to perform a  look-up in Active 
Directory of the mapped TSM server and communication parameters to use.

"A directory service provides a place to store information about network 
resources and makes the information available to administrators and users. 
Tivoli Storage Manager uses Active Directory to publish information about 
Tivoli Storage Manager servers. This information includes:
        The server’s name. 
        Communication protocols. 
        A list of the server’s registered nodes.

Tivoli Storage Manager servers running on Windows 2000 can automatically 
publish their information in Active Directory. By setting the ADREGISTER 
server option to YES, and issuing a REGISTER, RENAME, or DELETE NODE 
command, the server’s list of registered nodes is updated each time the 
IBM Tivoli Storage Manager server is started. Administrators can publish 
information about non-Windows IBM Tivoli Storage Manager servers with the 
IBM Tivoli Storage Manager Active Directory Configuration Wizard in the 
IBM Tivoli Storage Manager Console. The Active Directory Configuration 
Wizard is also useful for adding, removing, or editing Tivoli Storage 
Manager server entries. 

The Tivoli Storage Manager backup-archive client’s Setup Wizard includes 
the capability to browse server information in the Active Directory. The 
client can use the information to determine which server to connect to and 
what communication protocol to use."

Regards,

Mike Dile
Tivoli Storage Manager Client Development
Tivoli Software, IBM Software Group
mikedile AT us.ibm DOT com