> Perhaps I am just used to thinking of a database as a big interchangeable
> repository of data and not an integral part of the application code. Even > stored procedures are something I avoid.
Well, the feature is certainly less compelling if that's your orientation towards database systems. However, a modern DBMS is a powerful, feature-rich parallel execution engine, and some users like to harness that capability. If you don't want to use it, you don't have to.
The main benefit to this is that some users can eliminate the need to use a document database alongside PostgreSQL in mixed environments. This simplifies administration and provisioning.