I don't think it's really relevant to compare plain Postgres to Spanner, most folks have no need for something like this. It is made for folks who need to do millions of ACID type transactions a second/minute from all over the globe, and have a globally consistent database at all times.

There's a reason why Google installed and built their own atomic clocks and put them in their datacenters, it is to facilitate global timekeeping for this type of services. Most likely 99.9% of the time this type of database is overkill, and also likely way more expensive than you need.

https://cloud.google.com/spanner/docs/true-time-external-con...