What is the purpose of a spinal tap ? does it tells you what cause the bleeding in your brain?

A spinal tap can tell if blood has reached the spinal fluid, it does not divulge the etiology of the bleed. It can reveal blockage to the flow of CSF, inflammation in the brain and tumors if the tumor is affecting the surface of the brain.

Traditionally, lumbar puncture can confirm blood in spinal fluid secondary to vascular rupture in brain, or rarely spinal cord, but other studies are required to reveal cause or location. Other purposes are to monitor intracranial pressure, confirm multiple sclerosis, and to evaluate infections, such as meningitis or encephalitis.