What I understood (which I guess is wrong) is that Auschwitz II was made for extermination of every kind of person the Nazis didn't like. They arrived there, were selected on their health and the healthy were put on a train to the labour camps while the unhealthy ones were put into the gas chambers. Sometimes they had to wait for there to be enough prisoners to run the gas chambers, so I can imagine those prisoners being locked away for the time being.

Also they had prisoners working for them, are those put in the hundreds or camps? I cant imagine they need tens of thousands of prisoners doing the work, so why create the facilities to store tens of thousands of people when all they do is either put them in the gas chambers or send them off to different camps? Why not build more gas chambers if there are queues?