Root without password in Ubuntu

Since I use Hardy Heron, I found that we can get root console / command
prompt without a password.

I do not know, why Ubuntu team do this. But it was help me when I must
enter the admin functions but I do not know the admin user name and
password.

How to get in it?

When system is booting and prompt a GRUB menu, I press ESC key. Then I
select "Recovery mode". After the system show a selection action box, I
select "root". Than I get a sign: #