[Cushman]

Case law is still about fairness, though. The ideal is that the same facts should be decided the same way. Of course, since no case ever has exactly the same circumstances as a previous one, there's always some wiggle room to argue that this one is more like that one or the other one and so should be decided in that particular way. The adversarial system is a way to try to balance that; you make your best argument, I make my best argument, and somebody disinterested decides which one they buy.

Saying "you can use case law to support any argument" is hyperbole; you can use any case which was been decided the way you'd like in the past, and the older the statute the more cases have been decided and the more wiggle room you have. But it's not a binary thing; the strength of your argument is affected by how similar the circumstances are, how much body of precedent there is for and against, how close the jurisdiction was, the reasoning given for the previous decision, and many other factors.

Of course this is really hard to keep track of if you're not a full-time legal scholar in that particular area of law, and it makes the whole thing seem even more impenetrable to the rest of us. But again, it's all in the spirit of fairness. It's not perfect, but consider the alternative: a justice system where previous decisions didn't matter -- where the law was just "the law", decided fresh by whatever judge you happen to stand before -- would be far more capricious.