The main advantage of observational studies and natural experiments over manipulative studies is that natural experiments

The main advantage of natural and observational studies is that you see the experiments in their "natural" state - you are able to test hypotheses in the field and with large study areas There is no involvement of the person conducting the experiment which leads to the results unaltered. Any experiment that has outside influence of any kind can have skewed results because the person interfering whether directly or indirectly could cause the results to be biased in favor of the person or thing that interfering in a manipulative setting.