Personally, I'm not fully into D/S, but I consider myself the Dominant one because I have to be in control 100% of the time. I feel vulnerable if I am not in control.

Spanking is erotic and couples don't necessarily do it out of power control, but simply because the pain is stimulating and sexual. Some people do it as a punishment, but it's not to make you feel guilty, it's to please your partner and make them feel like a bad girl/boy in a good way.

I own a paddle and a riding crop and even using it on myself is found to be erotic, even though I doubt I can handle being spanked by someone else. I bought them to be used on a partner because there's a certain part of me that gets turned on to see someone moaning in pain and begging for me.

Michael D. said it perfectly.

That's my two cents.