Among the topics some rabbanim will discuss at the Shabbat Hagadol derasha is hechsher keilim, to teach how to remove absorbed taste from the walls of year-long utensils should we want to use them for Pesach. Much of the classical discussion of the topic is in regard to the laws of Pesach (see Orach Chayim 451-2). Before people could afford separate sets of just about everything, for the year and for Pesach, kashering for Pesach was an important rite of the season. The Torah talks about kashering utensils in two places. The most famous one is in Parashat Matot (Bamidbar 31:22-23), in the aftermath of the war against Midyan, when the army took spoils that included cooking utensils that needed kashering. The other one is in our parasha (Vayikra 6:21), in reference to utensils in which a korban chatat was cooked. Because the taste absorbed in the pot’s walls would become notar (holy meat that remained too long), the Torah writes that klei cheres (earthenware utensils) have to be broken and copper ones should be scrubbed and rinsed. Chazal learned from here that hagala (boiling water) does not work for klei cheres (see Rashi, Pesachim 30b).
Why did the Torah wait for these contexts to teach about laws that are so important in maintaining a kosher kitchen? On a halachic note, it is actually unclear if hechsher keilim is so critical on a Torah-level perspective. The gemara (Avoda Zara 76a) says that according to the opinion (which we accept) that non-kosher taste that spends 24 hours in the walls of a utensil does not prohibit food that was subsequently cooked in it, Torah law requires kashering only if one wanted to use treif utensils within 24 hours of their last use. Thus, before the Rabbis made things stricter, one could avoid most kashering, including for Pesach.
The Torah, though, requires kashering for holy utensils and discusses it regarding keilim acquired from non-Jews. Why is it noteworthy in these contexts? Perhaps the Torah is hinting at the following lesson. Keilim can technically have absorbed problematic substances, in which case, technical solutions (including waiting) suffice to solve the problems. However, utensils can also represent the activity with which they were involved. The Midyanite idolaters’ utensils represent their heathen lifestyle and the lack of concern for the purity of what they eat. They should be cleansed, both through hagala and tevilla (immersing) before we can use them for our activities. Just as the meat of kodashim needs to be dealt with diligently and not allowed to become stale and stagnant, so too its keilim need to remain fresh, thus requiring switching of klei cheres and renewing other keilim.
We live in a world with tools that can promote sanctity, impurity, or some combination thereof. We must ensure that ours are being used in a manner consistent with the desired result.