At the very least, reiki is simply placing your hands caringly on someone's body. Deep down, I think everyone craves human contact of an affectionate nature without ties and this is just that. In our modern, hectic world, it can be too easy to live without the plain, unadorned comfort of physical touch. Faith healing has been around as long as humans have. Shamans and witchdoctors have always called on the aid of mystic powers, just as priests have through prayer.
Priests derive it from their deity of choice.
Reiki feels great - especially if you are on the receiving end. Curiously, the healer's hands can feel icy cold to them, but burning hot to the recipient or vice-versa. Even if nothing out of the ordinary is felt, having someone undergo the ritual of resting their hands on your eyes, rubbing your feet, etc is very relaxing. In the late 1800s, there lived a man named Dr Usui who was a Christian minister in Japan. At one of his services, a senior student questioned his beliefs and asked for proof, not just blind faith.