They have one function and that is to increase positive caster, which is part of the front end alignment settings.
You probably know, or have at least heard after reading threads here, that Broncos until later in '76 had very low caster numbers. Lower even than they were supposed to have in most cases. It's rare that someone here reports having good caster numbers, especially after a lift.
This was not an issue back in the '60's and '70's with stock height and small tires and manual steering (more positive caster makes it harder to steer) but once we started adding larger tires and suspension lifts, the caster went down even more, often into the negative range.
Usually makes them a bit squirrely on the road.
Basically then even a stock height Bronco can often benefit from some additional caster. One method is to add offset C-bushings to tilt the top of the axle back.
When taller springs are added without dropping the rear radius arm mounts about the same amount, the radius arm design causes the axle to travel in an arc, changing the caster angle.
This is all better for the street and makes no difference for off-roading other than a better handling rig also handles better off road. But rock-crawling and even mud-bogging or dune running does not really feel much in the way of caster changes. Unless you have manual steering still where you'll likely find it harder to steer.
Tilting the axle only. with C-bushings or whatever, makes the pinion/u-joint angle worse. Hence the fact that 7 degrees has long been the most offset you can get in a bushing.
Poly-urethane bushings also clamp the axle tighter, which could be seen as an improvement. The resistance to flex should in theory help with body roll in corners (think of the radius arms as torsion bars) but probably limit suspension travel to a small extent. Not sure if anyone in recent years has bothered to check this difference, but it seems like it would do this to me.
Stock rubber is the better insulator/isolator from vibration. Poly holds in place better, often (but not always) lasts longer, the colors often look cool, and help with caster.
So depends on which of those features is more important