Well there are various reasons for that. Although as a "normal" user you can do almost everything in Linux (meaning the most common tasks such as writing CD/DVDs, watching movies, creating documents, etc)...
but if you wanted to change your mounted file systems, change the boot loader settings or just hate to use the Terminal all the time even for simple tasks like deleting files/folders where only the Root user has the ability (be careful)... and want to do all that without having to log into Ubuntu as the Root user?. Well then, this is how you do it!.
*. You have to be an administrator of your OS to do this.
Open your terminal and give the below command...
You'll be asked for a password, enter your user password for that (oh you already knew that ...) and hit enter. Within few seconds the Gnome file manager Nautilus will be loaded and you can do whatever you want with it.
*. Again be careful, the Root user has no restrictions whatsoever thus you can easily end up breaking your Linux distro!.
Oh and while you run the Nautilus as root, don't close the Terminal, otherwise it'll automatically close the Nautilus as well.