Obviously this is a role, and term of endearment, that is very common in the Ageplay world, and isn't about any biological relative. Many Innergirls crave a partner (whether romantic or not) whom they can think of with the special emotional attachments which the word "Daddy" brings to them.
Many of the traditionally-held attributes of good biological fathers mirror attributes which many Innergirls are craving in their life - reliability, strength, protection, comfort. It's common for an Innerkid to encapsulate those sorts of feelings about someone with the endearment "Daddy".
Of course one could fill many books (and many have been!) talking about the finer points and the origins of craving "fatherly" attributes in a lifestyle partner, and some women find too much discomfort with using a title which reminds them too much of their biological fathers, particularly those with Intimate Ageplay/etc relationships.
I've encountered a number of Innergirls who want a "Daddy" as part of a relationship because of diametrically opposite sensibilities burned into them from childhood. Sometimes, if their father was harsh or dictatorial or even physically abusive, the Innerkid feels a subconscious craving to re-experience their childhood with someone they've chosen this time and therefore trust, and who enacts even intense things like punishments. But this time she is comforted, cherished, and obviously loved.
So, an Innergirl's desire to think of an adult partner as "Daddy" can have all manner of sources and triggers, but I've never met any for whom using the word invoked any connection to their bio-father -- they crave fatherly attributes in an adult partner, without carrying forward any connection to their actual father.