At the end of the latter of this week's Parashiyot, Parashat Kedoshim, the Torah identifies the general purpose of the laws of Kashrut as (VaYikra 20:25) “VeHivdaltem Bein HaBeheima HaTehorah LaTemei’ah,” “You shall distinguish between the clean animal and the unclean”. Rashi comments that the Torah isn't coming to differentiate between a donkey and a cow, but rather between animals that were slaughtered properly and those slaughtered improperly. However, this raises a question. What’s the difference between a non-Kosher animal and an improperly slaughtered one? Didn't Hashem command us not to eat either of them?
Rav Moshe Feinstein answers that the difference between Kosher and non-Kosher animals is written outright in the Torah whereas the difference between a properly and improperly slaughtered animal was passed down to us through the Torah SheBeAl Peh, the oral tradition. This Pasuk is thus coming to teach us that we should be just as careful in our observance of the oral tradition as we are with the written Torah. The Torah's placement of this verse here, following a list of Arayot, improper relations, and not immediately after the earlier mention of Kashrut is also significant. Rav Feinstein says that this is written here to dispel the notion that Kashrut forbids certain animals due to health reasons. Some people may be inclined to think that the laws of Kashrut were enacted only because certain animals were unhealthy, and that if it is proven otherwise, maybe it would be permissible to eat a food such as pork. Though health may be a partial validation of Kashrut, it most certainly isn't the reason we observe it; rather, it is solely because Hashem told us to observe it.
Many similar forms of rationalization and reasoning have been used throughout Jewish history to explain Mitzvot. This has, in turn, led many to the misconception that the Mitzvot apply only when these rationales do, so Mitzvot can, in many instances, be incorrectly deemed as no longer applicable. That is why the Torah states both here and in other instances that we observe a Mitzvah due to Hashem's commanding us to do so, and not for any other reason. While many of the laws may seem illogical, like disqualifying an animal because of how it was slaughtered, these laws actually help us in our observance of the Mitzvot. Many of these confusing or illogical laws thus serve as a reminder that Mitzvot aren't to be done simply because they may or may not be beneficial to us, but rather because Hashem commanded us to keep them.