Why should I s-off when I can just unlock?

With an unlocked bootloader, you are able to flash and run custom recoveries, ROMs, and kernels. This is enough for many users. With s-off you have full control of the device. You can do things like flash radios and custom hboots. You can even change properties like CID and MID that can allow you to convert to different editions of your device. If you are a true power user, you need s-off.