IDE cables are designed to connect your IDE device to your motherboard or host controller. Round IDE cables are much more flexible than regular IDE cables and provide better air flow throughout the computer. Devices that use IDE cables can include CD-ROMs, Zip drives, hard drives, DVD-ROMs, CD burners, etc. IDE slots on the motherboard can only support up to 2 devices. Those cables are for cases or situations where you need to have a longer cable or to have some variety and options to place the devices where you choose.