My brother suggested a before/after. Here's the same model type, with an original slap on paint job, and then with washes, drybrushing, highlighting added. The differences are subtle, but together they make quite a difference, with easily visible changes being the face or cloth ankle wraps, and more subtle differences being the details on the bow, the fingers of the hand, and the highlights on the red cloth.
On the left miniature, all light and shade is simply light from the camera being reflected on the paint and surfaces of the model. So, depending on the light in the room, that miniature can have visible details, or be flat and devoid of any detail. The miniature on the right has the lighting and shading painted onto it, so the details are visible all the time, from any position.
This also shows how you don't have to paint a miniature from scratch to add detail to it. All my figures are just single-colour 'fill in the pieces' like on the left, but using a wash on those figures, then drybrushing and highlighting on top, performs an 'upgrade' without having to start all over with the figure.