Snapshots preserve the state and data of a virtual machine at the time you take the snapshot. When you take a snapshot of a virtual machine, the virtual machine is not affected and only an image of the virtual machine in a given state is copied and stored. Snapshots are useful when you must revert repeatedly to the same virtual machine state, but you do not want to create multiple virtual machines.
Snapshots are useful as a short-term solution for testing software with unknown or potentially harmful effects. For example, you can use a snapshot as a restoration point during a linear or iterative process, such as installing update packages, or during a branching process, such as installing different versions of a program.
You might want to use a snapshot when upgrading the operating system of a virtual machine. For example, before you upgrade the virtual machine, you take a snapshot to preserve the point in time before the upgrade. If there are no issues during the upgrade, you can choose to remove the snapshot, which will commit the changes you made during the upgrade. However, if you encountered an issue, you can revert to the snapshot, which will move back to your saved virtual machine state prior to the upgrade.
With vCloud Director you can have only one snapshot of a virtual machine. Each attempt to take a new snapshot of a virtual machine deletes the previous one.