An element management system (EMS) manages one or more of a specific type of network elements (NEs). Using an EMS a user can manage all the features of each NE individually, but not the communication between NEs.
Communication between NEs is managed by the network management system (NMS).
A network management system is a combination of hardware and software used to monitor and administer a network.
Network management refers to the maintenance and administration of large scale computer networks at the top level.
In brief, If network vendor wants to manage multiple EMS,vendor can go to NMS.
If network vendor want to manage one or more of a specific type of network elements (NEs), vendor can decide to go for EMS.