Futanari are not trannies. Trannies are victims of a mental disorder, that have been tricked into permanently mutilating their bodies, into a sad and vastly inferior imitation of the genitals of the opposite sex. A horror show, with numerous and major consequences. Smelling like poop. Permanent daily pain from dilation for male to female. Having skin cut off from their arm or leg, and mutilating their vagina to form a franken penis. One that can't get hard without an external pump. All of which sold to the victim, under the guise that they'll be happier in their "real" body. What would help them is to learn to accept the body they were born with. While well intentioned, positive promotion of surgery and hormone therapy, worsens the quality of their lives. Massive levels of cognitive dissonance are required in order to maintain the illusion.
Futanari are something that only exists in fiction. They are women, with the addition of a dick. They have the skeletal structure of a woman (unlike trannies). They have the smell of a woman. They have the femininity of woman. The cute facial features of a woman. The soft skin of woman. The collarbone and hips of a woman. Real boobs. They also have the mind and emotions of a woman; as opposed to a trannie's aggressive nature. The have the mannerisms of a woman.
The best futanari have both a dick and a pussy. They get to experience both. Less good, is where they only have a dick.
As far as the appeal, you get to see a woman struggle with having a dick. The shock and embarrassment. You get to see them get unwanted erections around other women, and panic while they try and hide it. If sex is involved, you get to see a woman's reaction to experiencing male pleasure. The appeal is the bizarre and alien nature of it.
The other appeal, is from a visual standpoint. You get to see penetration sex, without having to look at a naked guy. It's like lesbian sex, with a real dick attached to one of women. It's sort of still lesbian sex; and sort of not. But it's important that the futanari look feminine and still act like a woman. If the futanari is drawn androgynous or acts too much like a man, then it makes the whole thing pointless.
A tranny will always have that uncanny valley. To an observer, they will instantly know it's a man. Or if they are especially skilled in hiding their true sex, an observer will still have a feeling that something is wrong.