instead of the PSU having all cables connected to the back of it, only the ones that you require for your system can be plugged in, and the unwanted ones can be left in the box for future upgrades. As you only have the cables connected to the back of the PSU that your require, it reduces the amount of unwanted clutter in the case making it tidier and improving air flow. normally the only cables that are permantly connected are the 24pin ATX (which has 4 detatchable for older systems), and the 4 pin CPU plug.
hope that helps