Explanation
A snapshot is an image of your system/volume at a specific point in time. It captures the entire file system as it was when the snapshot was taken. When a snapshot is used to restore the system, the system will revert to exactly how it was at the time of the snapshot1. Snapshots are designed for short-term storage and fast recovery. They do not need a lot of storage space or time to create copies234.
Taking a snapshot of the system before the OS upgrade would ensure the fastest recovery of the system in case the upgrade fails in an unrecoverable way. The administrator could simply restore the system from the snapshot and avoid any data loss or corruption. This would be much faster and easier than performing a full backup or testing the upgrade in a preproduction environment.