“And these are the statutes that you shall place before them” 21:1
That you shall place before them – “Hashem said to Moshe, ‘Don’t think that you can simply teach the law to them three or four times until they’re familiar with it, without being bothered to teach them how to understand it.’ Therefore it says, ‘that you shall place before them,’ like a table that is set and prepared for a person to partake of.’” – Rashi
This was the first time the people would be hearing these Halachos, some of which would be logical and some of which would defy human reason. Hashem did not want Moshe to merely instruct us how to act, but also to explain to us how this code of law would benefit us to the utmost. Although comprehension of the rationale for mitzvos at the deepest levels often eludes us, and must not be prerequisite for their observance, there is much that we are given to understand and this was taught to us in tandem with the law itself. By insisting that Moshe include their explanations in the teaching of the laws, Hashem ensured that no Jew would ever be able to claim that he accepted the Torah unwittingly.