Like others said, using the term “Daddy” isn’t about visualizing my literal dad. (Shudder.) Rather, it’s about emulating a power dynamic between a (typically older) stern-but-loving guy and a (typically younger) sometimes-rebellious-but-ultimately-obedient girl.
People like these labels because they capture unique nuances about the dynamic. For example, the term “Daddy” implies that the guy is a provider, somebody who lovingly takes care of the girl. It also implies that the girl is relatively innocent. You don’t get that specificity with, say, “Master” and “Slave”.
Many people argue “Ew, doesn’t using the term ‘Daddy’ make you think of your father?” My counter is: No. I’m twenty-[x] years old. I haven’t called my father “Daddy” in like two decades.
You ever hear one of those 50s songs where the guy calls his love interest “Momma”? It’s sort of like that.