Hi Nnamjin,
GCE supports persistent disk snapshots with which you could backup the VM, in order to make them consistent the disk buffers must be first flushed. We need to prepare vSZ before taking that snapshot.
When this command is ran on vSZ this would sync the data to disk, take snapshot and resume back to normal, now snapshot on the GCE can be made.
I hope this answers your question.
Regards,
Abilash PR.