Well I did do legal studies, and I can confirm that Kini Mini is right.
Ratio decidendi is important, because a decision made by a judge sets a precedent which becomes part of common law, so future judges have to follow that precedent in their own decisions. But if a judge just made a ruling without stating why he is ruling in a certain way-future judges will not be able to follow the first judges reasoning. So the judge has to state the reasons he is sentencing the way he is...so if another case comes up with the same sort of factors in it, the second judge will have to follow the same reasoning as the first judge and rule in a similar way, because otherwise different judges could rule whatever they wanted. Precedents and the ratio decidendi prevents judges from making being creative or biased and making arbitrary decisions
Hmmm, that was a bit all over the place, I hope it made sense.