A Fitter-Welder 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-Welder under the Australian guidelines.
Many people want to move to Australia as a Fitter-Welder 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 Fitter-Welder, you will need to complete the Australian skills assessment for a skilled visa.