I can't even say whether or not Google benefits from Spanner, vs multiple Postgres DBs with application-level sharding. Reworking your systems to work with a horizontally-scaling DB is eerily similar to doing application-level sharding, and just because something is huge doesn't mean it's better with DB-level sharding.
The unique nice thing in Spanner is TrueTime, which enables the closest semblance of a multi-master DB by making an atomic clock the ground truth (see Generals' Problem). So you essentially don't have to worry about a regional failure causing unavailability (or inconsistency if you choose it) for one DB, since those clocks are a lot more reliable than machines. But there are probably downsides.
The unique nice thing in Spanner is TrueTime, which enables the closest semblance of a multi-master DB by making an atomic clock the ground truth (see Generals' Problem). So you essentially don't have to worry about a regional failure causing unavailability (or inconsistency if you choose it) for one DB, since those clocks are a lot more reliable than machines. But there are probably downsides.