Cerebrospinal fluid
Being surrounded by CSF helps the brain float inside the skull, like a buoy in water.  Because the brain is surrounded by fluid, it floats like it weighs only 2% of what it really does.  If the brain did not have CSF to float in, it would sit on the bottom of the skull.  The brain's weight would push the bottom of the brain against the skull.  Blood would not be able to get to the bottom of the brain because the blood vessels would be getting crushed by the weight of the brain on top of them.  Without getting blood (and the oxygen it carries), the neurons in the bottom of the brain would die.