A skills assessment is a mandatory requirement so you, the migrant, can apply for a visa to Australia. This is the way Australian Immigration can gauge if you are a Welder under the Australian guidelines. Many people want to move to Australia as a 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 Welder you will need to complete the Welder (First Class) skills assessment for a skilled visa.