Short lived, high frequency/throughput data (most likely some kind of non-essential logging), that gets accumulated on an interval in a different database would be such a case. Something like Redis can be the right choice there.
Also PostgreSQL is much more than a SQL database, it rivals an application server with its flexibility through extensions and supported languages.
Yes, but it has Append Only Files (AOF) that are similar to postgres' WAL and allow it to be durable. It can also fork periodically and write a copy of the database to disk. (It's single threaded and linux will COW when forking so it's safe and efficient iirc.)
Also PostgreSQL is much more than a SQL database, it rivals an application server with its flexibility through extensions and supported languages.