Veeam Backup & Replication requests vCenter Server or ESXi host to create a VM snapshot. VM disks are put to the read-only state, and every virtual disk receives a delta file. All changes that the user makes to the VM during backup are written to delta files.
The Veeam copy the VM data from the read-only VM disk, not from the snapshot. So why the snapshot needed during backup session? Is it a safety procedure if something goes wrong during copy data?
Snapshot is needed to freeze disk in time, otherwise disk blocks will change as we read them, which renders backup content useless.
That process is safe and is the way how VMware suggests backups shall be done - we read data from a frozen disk, and commit changes done on the machine afterwards.