Why is it important that prisoners are medically inspected when first received at Buna? What are the doctors assessing?

After the initial selection at Birkenau, Elie and his father enter Auschwitz and then are taken to the work camp at Buna. When they first arrive at Buna they are simply asked by the doctors if they are in good health. Since they have not been in the concentration camps for very long and passed the first selection by Dr. Mengele, they are assumed to be fit to work. This medical examination is mostly a formality. The doctors know that they are needed to work and that few of them will survive the harsh conditions anyway. The prisoners, however, are also examined by dentists, who are not interested in dental health, but rather in extracting gold fillings. Although he delays it, Elie eventually loses his gold filling. 

Later, after being at Buna for several months, another medical examination is slated by the SS. This time the men have been badly weakened by a lack of food. They survive on small pieces of bread and rations of soup. Elie writes,

An SS man would examine us. Whenever he found a weak one, a musulman as we called them, he would write his number down: good for the crematory.

Elie notes that his father had aged significantly in their time at the camp and the two are quite worried that he would be selected for liquidation. Miraculously, Elie's father survives this selection. Later, of course, he dies from dysentery at Buchenwald.