Besides these sections having to do with ritual purity, what is the underlying reason behind the study of these two chapters?
I recently learned Mikvaot for the MiYodeya 10-year siyum Mishnayot and noticed artscroll there explains
The study of the [seventh chapter of Mikvaot] is considered especially meritorious, because the first letters of the last four Mishnas spell neshama (soul) at the end of the chapter [R Yitzchak Isaac of Komarna]. It is therefore customary to study the chapter at a house of mourners during the thirty-day or one-year mourning period, and on the yahrzeit (anniversary of death).
Similarly on Keilim ch. 24 they write
It is worth noting that the present chapter is singled out from all the chapters in Mishnah as being particularly propitious for study in memory of a deceased loved one. This is because each of the chapter's first sixteen mishnahs ends with the phrase טְהוֹרָה מִכְּלוּם "it is pure from any form of tumah", and the seventheenth and final mishnah also concludes with the word tahor (pure) [Ta'amei HaMinhagim, footnote to Kuntres Acharon #1071]