Lately i have been seeing people paint black line on to their unpainted model, why do they do it? is it for refrence for the paint job ??? ??? ??? ??? ???
What you are seeing is called pre-shading. If you apply the next color carefully and try to leave some of that black exposed on the panel lines and nooks and crannies you create false shadows that will add interest to the surface of the model and create a finish slightly more interesting to look at.
The real trick is to not simply paint the model in solid color afterward and allow some of that black to bleed through, otherwise you completely cover it up and negate the pre-shade altogether. This is often a difficult balance to find at first but with practice can become quite easy.
If you look at the pictures below, you can see the dark pre-shade showing through on all the panel lines and areas of shadow, which was further enhanced by highlighting the panel centers with a lighter shade of color. It is simply an artistic approach to painting and finishing a model to impart a sense of scale.