Why do we have special haftarahs at this time of the year, like Parsha Parah?

The Chafetz Chaim sums it up best in Mishna Berura 685:1:

“Our sages decreed reading four special parshas a year from Rosh Chodesh Adar until Rosh Chodesh Nisan in remembrance of four events (Tractate Megilla 29).

They are:

1) Parshas Shekalim, to remember the mitzvah of the half-Shekel, which they were obligated to give to the (Temple) treasury for (purchasing) the Korban Tamid (daily offering) for the entire year.

2) Parshas Zachor, to remember the deeds of Amalek. We read it on the Shabbos before Purim to prop it against the actions of Haman, who was from the seed of Amalek, and to precede the remembrance of (our mitzvah of) erasing Amalek to our carrying it out (on Purim), as it’s written: “These days will be remembered and carried out…” (Esther 9:28).

3) Parshas Parah Adumah, on the Shabbos before Parshas Hachodesh, for then was its burning in the wilderness. (This was done) just before Nisan, in order that the sprinkling of the ashes of the sin offering upon Israel immediately follow the erection of the Tabernacle, so they would be spiritually purified and able to perform the Pascal offering in its proper time. Thus we read this parsha (in our times), praying before G-d that He should speedily sprinkle upon us the purifying waters.

4) Parshas Hachodesh, on the Shabbos leading to Rosh Chodesh Nisan, in order to sanctify the month of Nisan, as it’s written in the Torah: “this month will be for you the head of all months” (Exodus 12:2). But this is not the actual sanctification (of the new month), because the actual sanctification occurs at the time when the new moon is spotted and sanctified in Bais Din (the Jewish Court), whereas this reading is only by rabbinical decree.”

I suppose one may still ask: OK, but why don’t we read special parshas before other major milestones in our calendar?

Well, sometimes we do, like the special Hatorahs we read before and after Tisha b’Av. But the question of why we even change the Maftirs for the Four Parshas is still a good one. Perhaps it’s because Purim and Pesach represent two related geulas (salvations) of the Jewish People. In our era, awaiting our greatest and most needed geula, our sages saw fit to gird our resolve to pray and work toward our final redemption by reminding us of how G-d has always answered our call in the past.

That’s my guess. Let’s hope we’ll have the opportunity soon to ask Eliyahu HaNavi if I’m right.