A painted portrait is a complex combination of the sitter and the painter, the present and the future, the physical and the spiritual. It differs from portrait photography in that a painting presents the complete and timeless qualities, personality, and expression of the individual as opposed to a moment in time.
The finite image on a stretched canvas contains the infinite expression of a complex human being, and places them on a field of immortality.
The goal of a painted portrait is to capture the essence of the subject. That takes time and effort for the artist and the patron.