Helps to stabilize the camper and bed to the cab.
This same truck may have holes near the windshield where some more stabilizers were mounted to support the big Cabover campers.
Has absolutely nothing to do with keeping the front wheels on the ground.
Just helps to keep the frame from flexing in the center with the camper load on it.
We all know how much flex is in our pickups.
If not---park the truck with the left front and right rear wheels on a block.
Then you will see how much the cab and the bed move around.
With the cab and bed tied together----the camper will seem less topheavy, and the truck will not have wavy feel to it when hitting bumps or waves in the road.