I'm not a religious person, but I think Paul in Romans sums up why people benefit from BDSM training. He says "..glory in our sufferings, because we know that suffering produces perseverance; perseverance (builds) character; and character (generates) hope. And hope does not put us to shame, because God’s love has been poured out into our hearts through the Holy Spirit, who has been given to us."
Our modern lives are almost devoid of physical challenge, let alone suffering, and increasingly people protect themselves from exposure to even contrary points of view. The contemporary view is that people are fragile and are damaged by challenging experiences, when in reality the exact opposite is true. People, like our muscles and immune systems and many systems in society, strengthen themselves when challenged and atrophy when they are not. This shying away from adversity may be easier in the short term, but makes us more anxious and depressed in the long term, just as a lack of exercise may be relaxing for a day but leaves us weaker over time. This is terribly bad for both individuals and society, because as Bob Dylan noted, you don't want to be 'helpless as a rich man's child'.
BDSM is a way of challenging someone physically to build their capacity for pain and endurance. This goes on to build their character, which in turn gives them not only more capacity to face other challenges in real life but the ability to overcome them. The fact that it has a structure and a purpose protects the submissive from harm and gives point to their experience, and means they welcome the attentions of their dominent, rather than dreading it.
Good BDSM is essentially a type of Cognitive Behavioural Therapy, one of the few therapies with an evidence base of effectiveness. Someone afraid of spiders, for example, would be encouraged to get over their fear by gradually and facing their foe, gaining confidence from each small step to take another. You might be asked to write the word down, then look at pictures of spiders, be in the same room as a moth, observe small spiders in your house without freaking out and before you know it, you're playing with a tarantula - well, at least you can walk through a forest without trembling with fear.
BDSM is like CBT for challenge in life itself, and a skilled dom will use it to build their submissive's confidence, rather than destroy it. There are many bad doms who beat their partners to feel better about their own wretched lives by lording it over a helpless victim, but a real dom must love their sub, and know them better than they know themselves, taking their sub just a little bit further than they think they can go each time, but never too far. You must pay such close attention to the sub's needs - it's the sub who drives the process and so essentially controls the rate of progress - that this in itself is an intense experience, as we all need to be noticed in our lives, and a BDSM session makes us the centre not only of our world but that of the doms.
Spanking someone out of anger, or sexual aggression, or mere cruelty is never the point, but many of these things are masked by the label of BDSM. I think we should understand the point of it - both sub and dom - before we engage in it, and always remember what we are doing it for. It should offer us strength in our lives, just as pumping weights builds our muscles, and move us closer to God than the devil. This is also the reason why I think all doms should undergo the sub experience, not only to understand the type and severity of pain they might inflict on others, but to go through the same character building experience. If they do not, then the sub will soon become a stronger person that their dom, straining their relationship perhaps past breaking point.