Snapshot in Vcenter

Hello Experts,

I have Some query. As per my understanding Netbackup uses VSS for creating snapshot in vcenter and vcenter uses vmware tools to take the snapshot.

Pls correct me if i am wrong.

1:- What is the role of VSS volume shadow copy service in taking of the backup of Virtual machines in vcenter?

2:- why netbackup does  not use the vmware tools to take backup instead of VSS?

3:- Could some body tell me the concept of snapthot creation for Netbackup in vcenter?

Thanks,