Why does she want to be "punished" ??

I've learnt so much about various forms of domination and submission from these forums, but sometimes it still catches me by surprise when my girls says "Punish me.. I've been a bad girl.. I want you to punish me for everything I've done" and things of the sort, when things start getting hot and steamy.

It's still a bit of a blind spot for me, and sometimes I seem to "forget" why someone would want this. I know it revolves around being dominated, which I generally understand, but it's a specific type of submission and I want to be reminded again of the potential reasons behind why someone would use that particular saying ("punish"), why they would want to be "punished", and why they find it arousing.

Appreciate your help. Thanks!