postgres user account on OSX

There is a postgres user account on my OSX system. I'm not clear
about how it was created. I've installed a binary version of 8.3 in
/Library/PostgreSQL/8.3/ and built another version from source into
/usr/local/pgsql/. When I login as root and then 'su - postgres' it
takes me to the postgres account and the user directory is at
/opt/local/var/db/postgresql83/.

Can someone explain how this user account was created?

I'm trying to start the server that I built from source but it will
not create a logfile, ie:

elegans:~ postgres$ /usr/local/pgsql/bin/pg_ctl -D
/usr/local/pgsql/data -l logfile start
server starting
sh: logfile: Permission denied
elegans:~ postgres$
elegans:~ postgres$ nohup /usr/local/pgsql/bin/postgres -D
/usr/local/pgsql/data </dev/null >>server.log 2>&1 </dev/null &
[1] 28696
elegans:~ postgres$ -sh: server.log: Permission denied
elegans:~ postgres$
elegans:~ postgres$ pwd
/opt/local/var/db/postgresql83
elegans:~ postgres$
elegans:~ postgres$ ls -al ..
total 0
drwxr-xr-x 4 root admin 136 Aug 28 12:05 .
drwxr-xr-x 8 root admin 272 Sep 9 14:49 ..
drwxr-xr-x 3 root admin 102 Aug 28 12:05 postgresql83
drwxr-xr-x 3 root admin 102 Aug 26 13:06 smb

Should I remove this user somehow and replace it with a standard user
(using the system admin GUI)?

Thanks, Darren