Question Regarding Microsoft Windows Server 2012 Active Directory Domain Service

Hi
I am newbie with Windows Server 2012. I am trying to learn Active Directory Domain Services Role in Windows Server 2012, But i am not getting any satisfactory answer. can anyone explain me why do we use this role in production environment? If any one can give me example along with explanation,I shall be very thankful to you for this.

Note:- Mods if its not in proper section then please move it in proper section

Thanks
Sushant