What is a password for?

Dear all,

I am trying to create user/db under console with. I find that no matter I input a password or not when I'm creating or
dropping a user/db, the action can be success. What is a password for?

The following is what I have done:

~ $ createuser -U postgres -W super_creater_ctreated
Shall the new role be a superuser? (y/n) n
Shall the new role be allowed to create databases? (y/n) y
Shall the new role be allowed to create more new roles? (y/n) y
Password:<I input 'aaa' here>
CREATE ROLE

~ $ createuser -U super_creater_ctreated -W test_creater_created
Shall the new role be a superuser? (y/n) n
Shall the new role be allowed to create databases? (y/n) y
Shall the new role be allowed to create more new roles? (y/n) n
Password: <i input 'bbb' here>
CREATE ROLE

~ $ dropuser -U super_creater_ctreated -W test_creater_created
Password:<i input nothing here>
DROP ROLE

~ $ createuser -U super_creater_ctreated -W test_creater_createdno
Shall the new role be a superuser? (y/n) n
Shall the new role be allowed to create databases? (y/n) y
Shall the new role be allowed to create more new roles? (y/n) n
Password:<i input nothing here>
CREATE ROLE

~ $ dropuser -U super_creater_ctreated -W test_creater_createdno
Password:<i input 'bbb' here>
DROP ROLE

I cannot understand the mechanism for password. Is it a bug?

I'm using postgresql-8.2.6.

Weiwei