Barriers exist to prevent the world's mortal (and often souled), sentient inhabitants from knowing about the afterlife. For a creature with a soul, mortal life is meant to be a test that they are supposed to go through undisturbed until they receive judgement at their time of death and are sent to either Heaven or Hell.
With time the barriers have also come to serve as a shield against dimensional rifts that can create passages between Hell and the mortal worlds. Rifts allow demons and the like to pass through and can potentially break down the barrier entirely.
The arch angels and Heaven's government see it as one of their most important responsibilities to monitor and maintain the barriers. Rifts can be patched up by seraphim angels, but sometimes the damage is too extensive and the barrier is destroyed completely, making the mortal world into an open world.
Seperation of the mortal plane and the afterlife
A healthy barrier makes it impossible for creatures such as angels and demons to interact with the mortal world and its inhabitants. Mortals will be unaware of their presence and attempting to move or change anything on the mortal plane has no effect. Meanwhile a group of angels or demons can interact with eachother just like they can everywhere else.
The barrier can be weaker in some places, either because of a natural uevenness or because a rift is starting to form. In these spots it's possible for non-mortal creatures to affect their surroundings, resulting in "ghostly" activities. Demons have been known to take advantage of this to do physical damage, implant thoughts into people's minds with psychic magic, or otherwise haunt mortals.
With the appearance of an actual rift the barrier shatters, either in a contained area or across the whole planet. At this point there is nothing seperating mortal from non-mortal creatures and they can interact as if they existed on the same plane. It's worth noting that this doesn't change the mortals themselves - If you moved them to a different world with an intact barrier, they would be protected from Heaven and Hell once again.