Most of the belly pans and covers on undercarriages today are to benefit gas mileage. Better managing airflow especially in and around the non-aerodynamic chassis parts and assemblies can produce marginal mileage gains. Thus small gains here, small gains there, all contribute to the end goal.
Best case scenario would be to have air from the front fascia, bumper, grill move air to the sides, and over the hood with none going underneath. With the high stance of a truck that's simply not a reality. A best effort is made with minimal cost to deliver what we see today under our trucks.