Sample sizes can be larger and more significant that those obtainable within class or school.
Students encounter realistic challenges in designing, setting up, and carrying out experiments.
It all doesn't happen in one class period. There is also the risk of ambiguous results or even failure. Because results will be communicated to other schools, students have an added incentive to be careful and rigorous. Experiments can be replicated by other teams of student scientists. Students must confront sources of error and degrees of uncertainty in deciding what they have found out.