I see what a lot of you guys are saying it conveys submission, structure, protection, etc. Do women who had stable childhoods and strong fathers use it?
It's not necessarily limited to daddy issues. In cultures like Japan for instance, it's common for women to use a very high-pitched, childish voice when trying to attract a mate and when having sex.
Femininity is associated with purity and innocence; with a women's value being greatly connected with their youth and a limited age range where they can bear children; whereas, men don't have to worry as much in that regard, since they can retain their fertility even in old age, and losing virginity does not have the same biological impact. So such behavior can be seen as a more extreme way of appealing to the opposite sex in a subconscious sense.
Western cultures are more consciously-dominant in their thought processes of language. So when hearing the word "daddy", the first thoughts that come to mind are of a father strictly in the biological sense, which can be a turn off. But it is not necessarily meant to be expressed in a twisted way. Women's psychology is different from men's, they are generally more intuitive, and how they communicate information is not always going to be in the literal sense.
They may think that men find that kind of girlish act appealing, since they find it appealing in their minds, due to youth/children being of more significance and desirable in their lives. It is done subsconsciously, in an innocent, endearing way, and is just them freely expressing their emotions at a time where they can completely let loose and release tension without paying much mind. Even though it can be taken as creepy in a different context.