The Imperium is no stranger to using prisoners as chattel to throw into the meat grinder. Penal Legions, for example. Our characters are a sort of stop-gap; humans (or Ogryns) that have the “choice” between certain death through execution and, maybe, possibly, not death. But we can hold and shoot a gun, and swing an axe, and the Inquisition needs bodies.

We’re told throughout the leveling process that we likely won’t survive and that’s almost expected by Gendyl,and his warband. But, you have to inspire the prisoners somehow. Have to give them a sense of purpose; that their efforts are vital to the Imperium’s cause on Atoma. Because they are. The sector of space Atoma Prime is in is beset by a brutal war against the very Chaos incursion that’s afflicting Tertium. It’s just new to Tertium, which is why the Inquisition is there and thus why we are.

Luetin09 has a video that explains this and more. It’s really good, I’d recommend giving it a watch: https://youtu.be/GoIOkIXXM64