Hi
As what i'm aware of, spanking doesnt do any harm to one's physical well-being if you dont spank too hard like bleeding or something. In East Asia, like China, Korea, Japan, in the ancient time(a few hundred years ago) spanking with rods or whips was a common punishment utilized by the government, and from a medicine book written almost 2000 years ago in China in Han Dynasty which was also the dynasty when spanking was officially adopted by the government in Ancient China, It explained the reason for practicing spanking rather than other punishments which is "the butt is the thickest meat on one person and it is separated from visceral organs, so spanking the buttocks causes the least harm when using corporal punishments on one person." Besides, spanking is used as the most common corporal punishment THROUGHOUT the world, so you can easily deduct that spanking is not harmful.
For you second question, it's actually a very professional question and I only know a few about it. There are basically two big reasons why people enjoy spanking.
The first being physical, also the reason why many enjoy sadomasochism. When people are hit or hurt , some special chemical called Endorphin would be released in their brains in order to alleviate the pain, this being a very common and natural reaction most creatures demonstrate as for pursuing advantages of living., and it creates a feeling of being slightly anesthetized and therefore feeling easy and joyous. But like caffeine or morphine, some get addicted to the feeling endorphin enables them to feel so they are looking for chances to re-create the situation, which leads to the development of masochism, as well as spanking. PS. fun fact, some people love to eat spicy food, also because hot stuff stings their tongues to stimulate the brain to produce endorphin and it's absolutely enjoyable for them! XD
The second reason is mental or psychological concern. For someone who doesn't receive punishments growing up as a child, he or she naturally feels curious or even jealous of their peers who do. Often being people lacking parental care in most cases(in different ways), they want to attract attention and absorb care they should receive as a child. So they resort to being spanked again as adult to feel this "privilege" they enjoy.
this is all i know about why spanking is so hot to us…
hope it helps!