What is the motivation of include directive and configuration files outside PGDATA

Hello,

Let me ask about the background of configuration files.  I couldn't
find the relevant information in the 8.2 documentation.  I'm sorry to
cause you trouble.

In section "17.1. Setting Parameters", include directive is described.
Why was this directive prepared?  What usage is assumed?  Is it for
GUI tools, or for placing custom parameters in other files?

In section "17.2. File Locations", the following parameters are
described:

data_directory (string)
config_file (string)
hba_file (string)
ident_file (string)
external_pid_file (string)

What demand is assumed for placing configuration files outside the
data directory?  Is it for placing configuration files in /etc to
conform to the FHS (Filesystem Hierarchy Standard, as documented
below) to enable the backup of all configuration files on the system?

http://www.pathname.com/fhs/