A Volume Snapshot is a point in time backup of your Volume. It could comprise of a full backup and in addition an Incremental Backup. If you want a full recovery for your volume, then Snapshot is the best solution.
Using Volume Snapshot Feature, you can create a Snapshot of a volume which is in attached state or available state. If you delete your volume Snapshot, only the data copied over for Snapshot is removed, it will not affect your actual copy of the data.
Volume Snapshots are stored in Block Storage. When you create a volume from the Snapshot, the data is copied from the Snapshot to the volume. So, once your volume is ready, you can safely delete the Snapshot.
The same volume cannot be attached to multiple Instances. But using Volume Snapshot, you can replicate a given volume and attach identical volumes to multiple instances. Using Volume Snapshot, you can create multiple identical volumes.
Volume Snapshot is retained until manually deleted. If you delete volume without deleting volume Snapshot, the volume Snapshot will still accessible to you under Snapshot from the left side service menu and still chargeable.