You will need this certificate if you intend to obtain approval for work that was completed without a building permit or to demonstrate an existing building's compliance with any relevant standards. Building approval certificates are voluntary, but if unauthorised construction has been conducted, you may be required to demolish all works that have not been approved.
A building approval certificate can also be issued for the purpose of obtaining a strata title. However, if you intend to obtain a strata title, you cannot use the certificate to retrospectively approve works conducted without a building permit.