At speed, a few things are happening generally, and especially on a Dart of that era:
The recessed grille is packing whatever air doesn't get through (which also rams into the compartment & firewall).
(Venting the hood is commonplace among front-engine road-race cars, and now, the next-gen Cup cars.)
Air getting under the car is bouncing off all the undercarriage stuff, especially the ft suspension.
Air going over the roof becomes detached at the back glass & trunk lid, and roils (think an eddy in water), with the side-effect that it essentially pulls at the rear of the car.
A spoiler or wing essentially pulls the air more smoothly over the back glass & trunk lid, so that it leaves the car less disturbed with less effective pull rearward.
The Dart back glass is kind of a mini-version of the 2nd-Gen Charger, which had drag issues from the disturbance.
The idea is to get the air to go smoothly around the car, not through it (except for cooling considerations) or under it, and to depart smoothly behind it.
Spend some time watching airflow studies, or even water moving around rocks, and you'll see.

Fun fact: Adding a 4" angled spoiler to the back of the Melrose Missile picked up 7 MPH when it was stuck in the 150s.
The aero effect in that case more than compensated for drag.
Same thing added to a PS Camaro would make it more stable, but it would lose MPH.