Arivind,
Well it depends, what do you want it to do?
I've seen IDM used for:
SAP Role Provisioning
Self Service Password Reset
AD /. LDAP Provisioning
Exchange Provisioning
...
If you need your service desk to do these things, AD certainly can. As IDM implementations mature, IT/Service Desks are looking to have IDM be the central point for all user maintenance, reducing reliance on 3rd party and even native tools such as ADUC. The beauty of using IDM is that actions are trackable and auditable, plus you can lock down access to tasks in a way that you just can't with most other applications.
Does this help?
Matt