Why do we install active directory in the system?

in case you want:

securit, management, control, running some apps that require AD or if you have more than20 PC you should deploy a DC.

Active directory is a snigle database for users and computers and groups which give the network logical placement to get the user logged in any computer in the domain and control the pcs in the network by applying group policy 

Active Directory is an extensible and scalable directory service that enables you to efficiently manage network resources along with providing mechanisms for centralized management.