Creating a snapshot preserves the state and data of the virtual machines within a vApp at a specific point in time. A snapshot is not intended to be used for long periods of time or in place of backing up the vApp.
You might want to use a snapshot when upgrading the virtual machines in a vApp. For example, before you upgrade the virtual machines, you create a snapshot to preserve the point in time before the upgrade. To do this, you save a snapshot prior to upgrading, and then perform 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 the snapshot, which will move back to your saved vApp state prior to the upgrade.