Great question, thanks for asking. A parent snapshot in restic terminology is only used during backup. For an initial backup, restic needs to open and read all files that are to be saved, that’s unavoidable. But for a subsequent backup, restic tries to find a parent snapshot and then loads the metadata which was saved for that snapshot to check if files have been modified. Only modified files are read (and deduplicated) again, whereas for unmodified files restic just takes the contents the previous (“parent”) snapshot recorded. That’s much more efficient.
There’s also the --force option for the backup command which instructs restic to not use any parent snapshot at all and re-read all files.