“The purpose is to eliminate evidence.”
So the tunnels would be like large mass graves. The Nazis used mass graves for a while, and then switched to cremations. I guess the tunnel approach would serve a double purpose, a killing venue and disposal site in one. But who would they be hiding the evidence from? If foreign observers/invaders reached the site, they presumably would find the tunnels. If they didn’t, there is no one to hide things from. Maybe the intent is to hide the evidence from other prisoners. Or maybe they don’t have any other use for slave labor, so digging tunnels is a make work project.