Why did the SS officers conduct the selection in the book Night?

In chapter three of Elie Wiesel's memoir Night, he and his father go through the initial selection at Birkenau death camp to determine which Jews would be immediately put to death and those who would be kept (barely) alive to do slave labor. The selection was conducted by the infamous SS officer Dr. Mengele, who earned the nickname "Angel of Death." The selection determined those who were fit enough to work and those who would be of no use. The Nazis planned to basically starve the work prisoners, so they wanted only the fittest. They also planned on conducting medical experiments and looked for the healthiest. Those who were pointed to the right by Mengele went to their deaths, gassed and then burned. Those who went to the left were taken to barracks where they were stripped of their possessions and chosen for work details.

Before the selection, a man tells Elie and his father to lie about their ages. Elie is only 14 at the time but is told to say he is 18. His father, who is 50, is told to say he is 40. This difference in ages probably saves their lives. Still, there is suspense as it looks as though those who go to the left are headed to a pit of flames, just as Madame Schachter had predicted in her screaming fits on the train. But just before they reach the pit they are turned and pointed to the barracks. Once in the barracks they are further split up into groups. Those who are the strongest will work the crematoria, and others, like Elie and his father, are eventually taken to Buna work camp where they work in a warehouse for electrical equipment.

More selections are made throughout the book as the prisoners lose their health, and in, some cases, their minds because of starvation. A particularly important selection comes at Buna when some are left behind in the hospital and some sent to Buchenwald. At this point the Jews are able to decide their own fate. Elie and his father could stay in the hospital, where they would have been liberated by the Russians almost immediately. Unfortunately, Elie chooses to be evacuated, and eventually his father dies at Buchanwald.