A skills assessment is required so you, the migrant, can apply for a visa to work in Australia as a Plumber. The skills assessment is the way Australian Immigration can confirm that you have the relevant plumbing skills to match the Australian guidelines.
Even if you don’t need the plumbing assessment for the purposes of a visa, you will need it to become a registered Plumber in Australia as you require the OTSR to obtain your registration which is a mandatory requirement to work as a Plumber in Australia.