Why is there a shell given to user for PostgreSQL?

If a server admin uses sudo and is not careful about what environment and and umask this results in, working on the database may end up creating files in unintended locations or with unintended permissions.

Assigning a shell to the user enables admins to login as postgres and do the work on that users shell. Figuring out sudo as the source of unspecific server error messages would be too much of a headache.

If you do not need this, and are certain that you will never call postgres binaries in such error-prone way you can safely remove the shell:

usermod --shell /bin/false postgres

Keep in mind that, beign able to become root, you can still become anyone, including users without valid shells:

su --shell /bin/bash postgres

Authoritative source:

Sometimes you want to log in as that user to be able to do certain types of special administration or fixes. For example, if you ever need to run pg_resetxlog, you probably want to be logged in as postgres, unless you are very confident that your su or sudo invocations are correct and don't mess up the permissions of the database directory in strange ways. -- Peter Eisentraut, PostgreSQL dev

There is a shell because we use PostgreSQL from the command line as the PostgreSQL user.

Postgres runs under a special operating system user account for security reasons. This account is created on your machine when the installer runs, and unless overridden on the command line, it will be called "postgres".

On Unix-like operating systems such as Linux and Mac OS X, the account is setup without a password and users generally never need to worry about it again. Source.

Also it's not a good practice to edit the passwd file manually. You should use the command:

sudo passwd postgres