The main purpose of backups is to save the data of the virtual machine, and the snapshots — the current state of the VM. Creating a backup takes some time, during which the state of a VM can change. The system configuration is "frozen" when the snapshot is created. This allows you to save not only the files, but also the state of the VM.
It is recommended to create snapshots if you need to keep the state of the VM for a short time. If you want to save VM data for a long period of time, use backups.
Note: Restoring from a snapshot is impossible if the OS has been reinstalled on the VM or the VM disk has been cleaned. This is due to the fact that snapshot files do not contain a full copy of the disk, but only changes in its state. In such cases, use backup copies to save VM data.
Recommendations:
Before creating a snapshot, ensure that QEMU Guest Agent software is installed on the VM. If this software is not installed, the risk of inconsistent data in the snapshot is greatly increased.
If the VM is running a database or a highly loaded system, stop the VM before creating the snapshot. If stopping the VM is not possible, create a snapshot with the Allow saving RAM status option.