Mediums make the difference in how paints perform. Paints out of the tube are pretty much one-trick-ponies with tricks performed solely by the change in colour. Add a medium, and you can vary not only the colour, but add to the texture (viscosity) and change the colour-quality (opacity levels). As a glaze oil painter, working with transparency, translucency and opacity, the medium completely defines my working method as well as the product I am able to achieve. Since medium is actually the binder of the paint, it also defines your painting's durability. With a great deal of research, my choices are slim, but excellent. They work perfectly.