A skills assessment is required so you, the migrant, can apply for a visa to Australia. This is the way Australian Immigration can gauge if you are Motor Mechanic under the Australian guidelines.
Many people want to move to Australia as a Motor Mechanic and without the Motor Mechanic skills assessment delivered by an approved assessing body, Australian immigration have no way in checking a migrant has the necessary skills. To emigrate to Australia as a Motor Mechanic you will need to complete the Motor Mechanic skills assessment for a skilled visa.