Airflow thru the radiator (at speed) is exhausted back and down, onto/around the engine, under the body, then out the rear. That flow depends on positive pressure in the nose and negative pressure under the engine sides and rear. The chin spoiler helps set up the negative pressure, and I believe this skirt helps promote positive pressure flow directly onto the engine, rather than letting it dump out right at the front of the engine.
Part of this is for engine bay cooling, along w/fuel efficiency and wind noise reduction, I think, by ensuring a smooth flow in, under, around and behind the vehicle, vs. stirring, stalling, tumbling underneath that would increase resistance. I'm still amazed how quiet my 'Anny is regardless of speed. From flush fit glass to this humble skirt, it would be a shame to have such a wind cheating body shape just to kill the airflow underneath. Note: 700 hours of wind tunnel testing gave the 1989 Thunderbird a 0.31 drag coefficient (better than the '89 'Vette).