Why were striped pajamas worn in concentration camps in "The Boy in the Striped Pajamas"?

The title of the book The Boy in the Striped Pajamas comes from how Bruno sees prisoner uniforms at the Auschwitz concentration camp. Bruno's still too young and too naive to know what goes on inside the camp. He's blissfully unaware of the horrors that take place on a daily basis on the other side of the fence. So when he sees a young boy by the name of Shmuel wearing a prison uniform, he automatically assumes he's wearing striped pajamas.

Shmuel, like all of the inmates of Auschwitz, is forced to wear a uniform at all times to make it easier for the Germans to identify them. It's a way of dehumanizing them and making them all into one giant, amorphous lump, the better to be disciplined, controlled, and, when the time comes, herded into the gas chambers to be murdered.

But Bruno doesn't understand any of this. To him, this is all just a big game, an awfully big adventure. He wants to dress up in “striped pajamas” and join Shmuel in a little game on the other side of the fence. Shmuel manages to get hold of a prison uniform for Bruno, who puts it on and climbs under the fence. Mistaken for a prisoner, he is then sent to the gas chambers with Shmuel, and the two boys tragically perish along with many others.

Where do the striped pajamas come from in "The Boy in the Striped Pajamas"?

Shmuel brings the other pair of pajamas to Bruno.  When Bruno meets Shmuel at the fence, he finds that his friend is holding a pair that is “exactly like the one he [is] wearing.”  Shmuel has been able to obtain a pair because they are in abundance on the other side of the fence.  At this point, Bruno and Shmuel are no different physically from anyone else.  It is significant that Shmuel brings a pair of pajamas to Bruno because it is a moment in which "the other" merges with the majority in order to bring light to the idea that differences in which so much stock is placed are only external.  While both boys are demarcated as "different," Shmuel bringing the pajamas to make both the same.  There is no difference between them once Bruno dons the pajamas.  It is a point that ends up defining the end for both of them as they are herded into the gas chambers together.  The point being that while they are different, they go in together as few make any distinction in terms of who is victim and who is aggressor.  Bruno's wearing of the pajamas reflects how social constructs of identity can be temporal and, in this case, not telling of anything transcendent.