It seems that the reminder is necessary here because the bulk of the parsha discusses civil law. You might think that religion is all about ritual, about ceremony, but laws like not stealing, respecting someone’s property, etc. – that’s not religion, that’s common sense. Kah mashma lan….
What’s the kah mashma lan lesson to be learned here? The simple pshat is that the Torah is telling us that civil law is not just rooted in common sense, but is also Divine. That is why, as Rashi also tells us, you can’t go to a non-Jewish court even if they were to use the same rules and arrive at the same judgment as a Beis Din. And that also explains why Moshe had to be specifically told (Rashi explains) that it was not enough to just review these laws with Bnei Yisrael a few times until they knew the basic facts, but rather the logic and reasoning behind every law had to be explained as well. Moshe ordinarily needed no extra encouragement to teach Torah, but in this case one might have thought emphasizing how much sense the laws make comes at the expense of their appearing less Divine. Hashem therefore told Moshe not to worry and to just teach (Sefas Emes).
But there is another way you can learn the kah mashma lan (also in Sefas Emes, in the Likutim). Not that you might have thought these laws are just common sense and therefore not Divine, kah mashma lan that the laws are in fact Divine, but rather you might have thought these laws are common sense and therefore not Divine, kah mashma lan that your common sense and reason is also Divine. The only difference between parah adumah, for example, and not stealing, is that in the former case G-d has not opened up your brain to the possibility of understanding the law but in the latter case He has.
The philosophical question of whether something is wrong because G-d made a law saying so or whether G-d made laws to stopg us from doing wrong doesn’t get off the ground according to this approach. The question presupposes a false distinction between what G-d declares and what we know innately, but in truth, there is no “I” that knows not to steal if not for the fact that G-d made me so.