Tetzaveh: Darash Moshe: Mitzvah 102 – Concept 320
“They shall attach the Breastplate from its rings to the Ephod with a turquoise woolen cord so that it will remain above the belt of the Ephod, and the Breastplate will not be loosened from upon the Ephod.” Exodus 28:28) The Breastplate must not be loosened from the ephod. (Rambam, Hilchot Klei Hamikdash V’Haovdim Bo – The Laws of The Temple Vessels and Employees)
The Sages understood this to be a negative commandment prohibiting the removal of the Breastplate from the Ephod. (Yoma 72a) To understand the rationale of this commandment, let us remember that each of the priestly garments atoned for specific categories of sins that the people might have committed.
Specifically, the Ephod atoned for idolatry, while the Choshen atoned for perversions of justice, both those of individuals in their business dealings and errors made by judges in their rulings on cases that came before them . (Zevachim 88b)
Transgressions of the Torah’s monetary laws at both of these levels reveal a weakness of faith in Hashem; anyone with a strong conviction that Hashem provides for everyone’s needs will make it his highest priority to avoid violating even the least stringent of the Torah’s monetary laws. Thus, laxity in observing these laws is tantamount to denying Hashem’s power, a sin, which required the atonement of the Ephod.
This is why the Torah, in its wisdom, required that the Choshen and the Ephod be inseparably joined, since sins that fall into the purview of one of them inevitably involve the other as well. (Darash Moshe)