It does not only portray a person or the painter's interpretation of that person but how the artist himself wishes to be perceived or sees himself. It adds a level of complexity which I find fascinating. It can be argued that how someone looks is literally skin deep and that is in a sense true, but in my opinion it still tells you a lot about a person. I'm not talking about raw beauty as that is certainly not something we have any control over but how people hold themselves and how they come across when you interact with them which is far more interesting.
That is of course the challenge for a portrait painter - to capture the essence and the feeling. These days one might think that it's a dying or at least no longer relevant art form but a good oil portrait often tells you much more than a photograph ever would. You can for example clearly see if a painting has been done completely from a photograph and it generally means a certain flatness. Contemporary artists Vanessa Garwood and Jamie Coreth who's paintings feature below paint almost exclusively from life and you can really tell.
Let yourself be seduced by portraits of the past and present, feel how they absorb you into their sphere and teach you something about human nature - like any good art should...Jamie Coreth, Portrait of Beata Heuman, 2016