Meninges
One of the arachnoid layer's most important jobs is to cushion the brain.  Like the pia mater, the arachnoid mater is made of cells that fluid cannot get through.  Because fluid cannot get through either of these layers, the cerebrospinal fluid stays in the subarachnoid space and does not leak out.  Having this layer of fluid helps protect the brain, like a car's airbag.  Without this cushion, every time a person moved their head, their brain would hit the inside of the skull and would get hurt.