Phppgadmin includes instructions for developing a reviews database around the system to be used with phppgadmin. The instructions describe how to put it together, but don't really give any suggestion of the items its purpose is, and also the phppgadmin site was not so useful either.

It appears to let you store SQL queries, so will it be for storing admin queries being able to access tables like pg_class etc?

Case a typical place to store commonly used SQL scripts. The reviews-pgsql.sql script produces a table for storing these queries, the database they usually are meant to be operate on, a title plus some descriptive text about the things they're doing. PhpPgAdmin has functionality to browse and execute these reviews. It is a really quite simple system just meant to assist in organization.