Why is a person with increased intracranial pressure given the diuretic mannitol?

Mannitol: Mannitol works to reduce intracranial pressure through its diuretic properties. It is usually used in the acute setting (after head trauma).

Osmotic diuretic : Mannitol is osmotic diuretics so by giving mannitol you remove fluid from body and that also help reducing edema of the brain. As increase brain edema is most of the time responsible for increase intracranial pressure, reducing edema help to lower it.