The Joiner skills assessment Australia 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 Joiner under the Australian guidelines.
Many Joiners want to migrate to Australia and without the Joinery 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 Joiner you will need to complete the Joiner skills assessment for a skilled visa.