Registration protects everyone against fraud and theft, as well as ensuring that vehicles are insured for any damage they might do to other people in a crash. You need Compulsory Third Party (CTP) insurance before you can register your car in NSW.
Besides the green slip, you should consider what other car insurance cover you want. There are different levels, from the basic Third Party Fire and Theft to Comprehensive, which covers a large range of situations. Use these articles to guide you on your registration and insurance requirements.