The justice system of any country would be lost if there was no way to keep a check and balance on things. In fact if there was no way to hold those in powerful positions accountable, it would be very easy for people to abuse their power and for them to become corrupt. While not every person would do this, some would still go ahead with illegal things and they are enough to cause a lot of problems for the common people.
Transparency and accountability is important in the justice system otherwise there would be no point to the courts, the judges, and the juries. People could simply win based on who they know or if they were able to send a few bribes that got accepted and no one would be the wiser. This is why even for courts there are other justice systems that can review them and the decisions that they make to ensure that no person is unrightfully being put in to a jail. To make sure that lower courts are not acting in illegal or illogical ways, there are appellate courts that are going to take care of them by reviewing their work. One special type of review or appeal is the abuse of discretion review or the de novo case appeal where the history and actions of the court are taken in to consideration.
This sort of appeal does not look over the evidence and the decision that was reached by the court and then try to rework it to a different conclusion. Instead it checks whether the court has been complying with all of the legal regulations, whether they are reaching logical solutions, and whether they have a history of such appeals. You can go online to http://www.calblogofappeal.com/2014/06/11/sometimes-the-standard-of-review-is-better-than-you-might-first-think/ and learn more about this sort of case.