The ARBs have a number of effects on the car and watching the telemetry will highlight them. (Note: I have found some none realistic results in the telemetry, so use is as a guide, not as absolute truth. )
The primary function is to keep the car from leaning so much from side to side. Hence the term "anti-roll" This has a secondary effect of making the car more reactive to steering input. Since you don't have to wait for the weight of the car to shift from left to right , or vise-verse, going through chicanes is snappier. The delay between turning into a corner and the car actually turning is less. Another effect is that the car won't roll to the side as much, during long or high speed corners. Since the right and left sides are more rigidly connected , with higher ARB settings, the inside tire would have to lift off the ground, to roll to much. If you watch some of the actuall F1 coverage, you will see the inside tires lockup sooner, or stop rolling entirely in sharp corners. That's becaus they run so stiff that the inside tire gets very "unweighted" in the corners.
Ah, but there is a bad side. Since a stiff ARB makes the weight of the car work harder on the outside wheel, the car will have less grip in the corner and will understeer. It will also wear the tires out faster, since the work is not balanced between right and left.
How well that GP4 actually models this is up for debate. What you try to find is a setting that is agile enough for your driving but doesn't understeer.
JohnW