Only what is law is binding. But is every law also just?

Not necessarily. Chance and self-will can slip in when someone writes the law down. Then a law can say something other than what is right in itself.

What does legal science do?

It asks what holds according to the laws. It appeals to authority and history. It orders and applies. That is its task.

But it must not take offense when someone asks: Is this rule also rational? This question belongs to philosophy. A rule can have a long history and still be irrational.

Summary: The law is binding. But law and right can part ways. The question whether a law is rational remains justified.