看来它没有真正有一个明确的目的。根据文档：
Creating a database cluster consists of creating the directories in which the database data will live,generating the shared catalog tables (tables that belong to the whole cluster rather than to any particular database),and creating the “template1” and “postgres” databases.
[…]
The postgres database is a default database meant for use by users,utilities and third party applications.
(来源：http://www.postgresql.org/docs/8.2/interactive/app-initdb.html)