Usernames and passwords may seem trivial if you are working with your own system. However, there is no mercy in the UNIX and Linux (*NIX to be short) world; you are not asked a hundred times if you are sure to make a change: you issue the command and it is executed. In order to save yourself from making important changes mistakenly, you need to know what you are doing with proper privileges.
In *NIX systems, super user is the God of the system, having the rights to destroy everything. If you can imagine what can happen if your password is in another persons hands, pay attention to your passwords; make them strong, keep them safe and change them from time to time.