PostgreSQL is an advanced Object-Relational database management system
(DBMS) that supports almost all SQL constructs (including
transactions, subselects and user-defined types and functions). The
postgresql package includes the client programs and libraries that
you'll need to access a PostgreSQL DBMS server. These PostgreSQL
client programs are programs that directly manipulate the internal
structure of PostgreSQL databases on a PostgreSQL server. These client
programs can be located on the same machine with the PostgreSQL
server, or may be on a remote machine which accesses a PostgreSQL
server over a network connection. This package contains the client
libraries for C and C++, as well as command-line utilities for
managing PostgreSQL databases on a PostgreSQL server.

If you want to manipulate a PostgreSQL database on a remote PostgreSQL
server, you need this package. You also need to install this package
if you're installing the postgresql-server package.