Whats is VTP? why we use vtp in cisco switch?

cisco VTP proprietary protocol used to reduced switched network managment and administration, it distrubute the vlans through all switches in the domain (VTP domain), which reduce the need to configure the same vlan everywhere.

you can refer to cisco documentation for more details.

VTP is a Layer2 Cisco proprietary protocol for managing Virtual LANs. VLAN Management will be very efficient using this protocol. But conflicts may occur because of different network designs. Loops may occur and may mislead traffic. Then another protocol may be used, STP.