While polaroids are done for evaluation of model physique in general, model test is done to show how much model is loved by the camera. While some people are very pretty in real life, they might not be photogenic at all, and vice versa, looks that feel quite boring and dull might shine when image is taken. Quite a lot of that depends on time and effort that model has put into finding her best angles and presentation of them to the camera and the rest is "god's particle" impossible to fake. So the ultimate goal of model test is to make a simplistic but yet creative photoshoot to show that.
Nikon D3s Nikkor 50mm 1,4G @f/8 1/250 iso200