Law can benefit tyrannical regimes by making the things that they do seem legitimate. Of course, the laws in such regimes are not made in democratic and legitimate ways so the appearance of legitimacy is a sham. Even so, the presence of the laws gives tyranny a more normal appearance.
If tyrants simply do whatever they want, it is clear that they are being dictatorial and tyrannical. However, if they put laws in place, it does not look that way to the same degree. They can simply say that they are following the laws, just like governments are supposed to. They can even have judges hold trials and hand down verdicts just as is done in a democratic society. By doing these things, tyrannical regimes can make themselves look good, thus making it more likely that their own people will support them and that the international community will not condemn them.