On the outer layers it is called "Copper Thieving" or "Plating Balance". It is added to the outer layers of a pcb to create a uniform distribution of copper across the surface. This is done to make sure that the copper plating in the holes is uniform. If the copper distribution in the artwork is not uniform, areas with little exposed copper will plate very heavy while areas with large amounts of copper will not plate properly. So, “thieving” steals some of the plating current that would otherwise concentrate on features that are sparse and be spread thin in areas with areas that are dense with features.
On inner layers they are added to even out the resin flow from the prepreg.