For big projects with big power needs, an AT or ATX power supply is a handy place to grab 5V or 12VDC. The only problem is that these supplies have connectors that only plug into motherboards or hard drives. That's where this cable comes in. It has a matching 4-pin Molex connector to the hard drive/CD-ROM power cables that come out of a power supply so you can use this to grab 5V or 12V. Heck, you can even use this cable while your computer is powered, say to decorate your hard drive with LEDs or VFD/LCD displays, etc.