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 in fact a Fitter under the Australian guidelines.
Many people want to move to Australia as a Fitter and without a 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 mechanical Fitter you will need to complete the Fitter (General) skills assessment for a skilled visa.