Lets say you have five identical computers you want to install Windows XP on. You can setup some scripts that will set all the settings the same, install all the components you want, configure the network the same etc. Then you start WinXP setup on each machine and walk away. You don't have to hit 'Enter', type in your CD key etc. etc. You just walk away and come back in an hour or so.
EDIT: Forgot...
So when you come back from your lunch break, the computers are all booted up running XP in OEM preinstall mode. At this point, you can install any additional MS software or 3rd party software (Office, Wordperfect, Adobe stuff, Macromedia stuff etc.) or setup additional drivers that XP wouldn't have. You can even run all the Windows Updates. Then when everything is done, you closeout OEM preinstall mode by clicking a button and the machine shuts off. Then, when your customer starts the computer, it boots into Windows but the first thing they see is a dialog asking for their name, initials user name, cd key, activation etc.