Many professional fantasy artists hire models too, if they're not already using their own friends and relatives to pose. Well, the rest of us can't afford models, or maybe our friends are too bashful, or we don't have the costumes, or a camera, or a large mirror. For the rest of us, copying figures from 2-dimensional sources is a cheap and easy alternative. Some illustrators compile scrapbooks. Some animators cover their walls with visual references.
Some artists take landscape photographs specifically for their paintings.
Rembrandt made copies of Leonardo's paintings and incorporated the studies into his own paintings. Copying is a means of learning. You learn how to look at and render the human figure, fabrics and armor. You can study the details in different textures, forms, shapes, materials and shine in objects. You can take note of the colors and compositions used in other pictures.