It's a basic principle of justice that police must reveal in court how they obtained evidence. Otherwise they may have obtained it unlawfully. At the same time, defence lawyers must be able to examine and challenge the methods used to obtain the evidence. Not only could the method have obtained evidence unlawfully, but the evidence itself could be faulty or misleading.

Police don't just get to say of evidence, "This is the real thing." They have to be able to demonstrate why that is so.