People think of all paintings as being flat and light them accordingly. Nothing could be further from the truth, especially with paintings done in oils or acrylics. 19th century painters began laying on paint thicker in certain places than others to help enhance the feel of their work.
This is called impasto.
When photographing a painting, if the photographer does not bring this out with his lighting, much of the artist's intended impact is lost. It can be overdone, but with restraint, it reveals secrets not otherwise seen.
Click on these images to see masterful examples of Impasto. If these had been photographed conventionally, with two lights at a 45 degree angle, most of this wonderful texture would be invisible.