Snapshots are a means of getting around open files. If the data you're considering backing up is part of a database, such as SQL data or an Exchange messaging system, the the data is probably being used all the time. Often copy-based mechanisms will be unable to back up open files. Short of closing the files, and so too the database a copy-based system will not work. A snapshot is a point-in-time copy of data maintained by the file system. A backup can use the snapshot rather than the live data to perform the backup.
Virtual system managers can usually take snapshot or cloned copies of VMs.