The Torah reading of Parah (Numbers 19) is added to the weekly reading. Parah details the laws of the "Red Heifer" and the process by which a person rendered ritually impure by contact with a dead body was purified.
(When the Holy Temple stood in Jerusalem, every Jew had to be in a state of ritual purity in time for the bringing of the Passover offering in the Temple. Today, though we're unable to fulfill the Temple-related rituals in practice, we fulfill them spiritually by studying their laws in the Torah. Thus, we study and read the section of Parah in preparation for the upcoming festival of Passover.)
This they shall give . . . half a shekel (30:13)
The mitzvah of the half-shekel is that each should contribute a coin that [is valued at] half of the dominant coin of that time. If the prevailing coin is a takal, they should give a half-takal; if it is a sela, they should give a half-sela; if it is a darcon, they should give a half-darcon.
(Midrash HaGadol)
Why not a complete coin? To teach us that no man is a complete entity unto himself. Only by joining with another can a person become a “whole thing.”
(The Chassidic Masters)
https://w2.chabad.org/media/pdf/12293.pdf