My partner and recently adopted this euphemism. And I was just curious as to why this would arouse us both. It felt odd but we really liked it.
So I took to Google, where else?

The following content below is not mine it is just copied and pasted.
and by no means is any infringement intended.

Traditionally, the male is viewed as a provider, one who takes care of his woman.

This is not unlike the relationship between father and daughter.

The primary difference is that the father has significantly more authority over his daughter than does a man over a woman in the relationship. The daughter is seen as young, naive and needing guidance.

By encouraging his woman to call him “Daddy,” a man is essentially making himself seem stronger, and more powerful. The man loves having the woman utter a phrase which is reminiscent of the power differential in the father daughter relationship. The word suggests that the man is totally in control – and the man loves this feeling of total control – and the woman is in harmony with this feeling of being temporarily subservient to her man. It creates a feeling of a powerful differential. To them, that is hot.

Additionally, the phrase connotes youth. By saying “Daddy,” the woman is in a way implying “Hey I’m young enough to be your daughter.” Youth is attractive to men, and women want to feel youthful.

Additionally daughters are innocent, so it creates the idea that the man is enjoying someone who is seeing the world with open eyes, someone who is likely a good person because they have not yet been corrupted by the evils of society. Some women want to seem good. Some men love the idea of a good woman.

Beyond that, some men like to feel like they are good people, and one of the ways they do that is by taking care of someone. If a woman calls a man “Daddy,” he feels like he is taking of her as he would a child. This fantasy that he is a better person than he actually is can be appealing. Similarly, some women, especially those who may not have received enough parental attention in childhood, love to feel like they are taken care of – and that they have a surrogate parent. Therefore this type of interaction could appeal to them as well.

There is also the forbidden-ness and taboo of it. That can add excitement.