Both my '85 3/4t 4x4 diesels came with them, as did the '90 3/4t 4x4. Just picked up a front bumper from a '90 gasser Sub, which has an abbreviated version, doesn't extend all the way out to the sides, and is also more of a "chin spoiler", sticks out forward at an angle, whereas the diesel versions are vertical, pointing straight down at the road.
I recall a comment about high pressure tending to accumulate under the hood on these trucks... maybe more on the 3/4t models since they sit a little higher? I'd expect higher underhood pressure to lower airflow rates through the grille and radiator stack. Not sure... but I'd guess that this would be the reason for the spoiler. Aside from a slight help with mpg, I can't imagine why else they'd have done it.
PS: The gasser dam definitely looks a bit racier than the stodgy diesel design, at least on the OBS trucks.