Why do you do purging in crude oil tanker ships?

The tanks are cleaned with a machine called a Butterworth or Gamajet. Regardless of the actual manufacturer they are almost universally called Butterworth Machines.

These machines shoot hot crude oil onto the sides of the tanks so that it washes the sediment down to be pumped out.

The tanks must be cleaned before they can be inspected, and before cargo of a different type is loaded.