Why? If an artist only paints from a printed image they can not experience the elements.The curve of the cheek, the reflected light that bounces off the vase from the flower above, the way the air affects the colour of the green in the landscape.
I remember the first time I was plein air painting. Oh my goodness. How overwhelming! Everything was there. I did not have the edges that my photographs gave me. I had so many decisions to make. Then there was the first time I had a real figure in front of me. Not only had I not experienced how truly beautiful every body is but how complex the angles were and yet how symmetrical each of us are. And yes I drew still life in high school, but not with the knowledge of composition, value, light and balance. Where was to start.
There is the point to start. To make the decisions. To view what is before you and to try to depict the image on your canvas. For all the first timers, that is what you are doing. Learning to depict what you see. If you come up with your master piece then you are a master. I can not tell you how many bad paintings I have had from these experiences. But here is the positive. I began to see. I began to see how light never sits on the edge of a curve, because if an object is round, as it curves it is now heading away from the light. I began to see that even though the trees were the same colour as the ones that were closer to the horizon because of air and atmosphere they changed in value. Oh and the reflected light, our image catchers don’t get it.
I am not saying that you must work from life all the time, what I am saying is that if you experience the elements in real life when you work from your printed images, you see beyond them and paint what you know is real.
Now you are on the road to being an artist that expresses not only what they see but what they know.