It is the duty of researchers to obtain ethics approval before commencing a study.
Ethics review by an experienced and independent team of experts will reassure researchers that they have everything they need to safeguard research subjects from possible harm.
When ethics review is not sought and a study subsequently leads to harm / complaints / litigation, researchers will find it much more difficult to defend themselves against any claims of malpractice or negligence.
Besides, research funding agencies often require evidence of ethics approval before they would fund a study. Likewise, a study that has been conducted without ethics approval will often be rejected from publication by a scientific journal.