In a positivist conception of law or in in how it is taught , we tend to think that the history of law is an "auxiliary science". But if it happens that past events remain in the current law, so it is ignoring the current law to know nothing of the history of law.
If we take the example of the French Revolution, a major divide in the French history, is there still something in the French law?
If not, then let the knowledge of the Revolution to historians.
If so, then the practical handling of the current French law requires knowledge and understanding of the French Revolution and its ideas, which are still working through today's law, for example through then texts still preserved in its letter and in its spirit.