The multiple layered defence is valuable just by existing because it prevents deep attacks and maneuver warfare.

When you have just one super fortified line, you can break that line by creating a threat behind it that makes the soldiers want to fall back to a safer position (they all want to live at the end of the day). By having multiple lines like this just existing you prevent the ability to drop troops behind the enemy lines, such as with paratroopers or helicopters. You prevent the ability to set up a pressure coming from another direction that causes a redeployment pressure on the first line.

In WW2 a major part of the operations was paratroopers landing behind enemy lines in order to threaten the enemy defensive lines preventing landings from another direction, causing those lines to fall back to other positions.

It's not actually about killing people and "winning" the fights, it's about creating pressures that cause people to move their lines.

The way these defensive lines are all set up, their existence prevents deep warfare like this. They don't have to do anything at all other than exist to force the enemy out of using that tactic. Crude drawing of this pressure, the blue triangle being the deep-offensive attacker and the red line being the natural line of redeployment sought by the defenders if they're under these kinds of pressures:

You can't do this shit if there's multiple lines of defence that would then function applying exactly the same kind of pressure on your deep attacker.