I paint from life. The colors of the model are richer and the sense of space or the third dimension is stronger if the artist paints from life not a photo. Also, the model settles into the pose and her/his character, or soul, surfaces. My teacher at the National Academy, Louis Bouche, used to say that the model paints the mood.
If the painting is of a scene including people, I will sketch on site and photograph, but then I change some of the poses to give a sense of movement and contact between the figures. For instance, in “Bleecker Street” the young woman in the foreground faces into the picture on a diagonal to emphasize the depth. She is not isolated; she looks at the man with the baby. My favorite model at the time posed for the young lady in the foreground.
I should add, my many prizes were for figure paintings.