Even if it's never shared, it can be used by the person to enhance cognitive distortions of the person who is depicted. As well, if you have images of a lot of different women, for example, that can help to make you feel like women are available for you in whatever sort of context you might be able to dream up.
I think there's harm to women and girls generally in terms of the existence of these images, even when they are held for private use.