Hello Roshan
The purpose of the two types of servers (secondary and primary) are to improve redundancy. Specifically, a secondary server stores the received configuration in a local permanent storage space (for example, NVRAM) and updates other devices in the same domain and for the same instance. In the event that the primary server fails, a secondary server can be promoted to be a primary server. More information about this procedure and its benefits over VTPv1 and v2 can be found in the following Cisco Documentation:
I hope this has been helpful!
Laz