You don't necessarily need snapshots on a watch, but I can see it coming in useful on a phone when you're editing pictures and video.
Snapshots on a watch could be useful for system recovery. Say something went wrong during an update you could potentially use the snapshot to revert to a previous version.
On the other hand, computers are gradually moving towards miniaturization, so I suppose all this really is quite transitory, and soon enough all such considerations wills simply be irrelevant.
The main point is that upgrading "in-place" does not require any of the metadata to be in the same place between old and new filesystem. With a mildly flexible destination filesystem, the old filesystem can stay there until you're completely sure the conversion is a success. You end up with two read-only filesystems sharing a partition, and you choose which one to go forward with. Cancelling at any point is trivial, even if the conversion process crashes.