The skills assessment is required so you, the migrant, can apply for a visa to Australia. This is the way Australian Immigration can gauge that you are Sheet Metal Trades Worker under the Australian guidelines. Many people want to move to Australia as a Sheet Metal Trades Worker and without the Sheet Metal Trades Worker 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 Sheet Metal trades Worker you will need to complete the Sheet Metal trades Worker skills assessment for a skilled visa.