Musar su Osea 3:4

ולא יזח החושן מעל האפוד. Rabbi Menachem Ha-Bavli writes about this verse that the reason for this commandment is analogous to the principle that anyone who appoints a judge who is unworthy of the position is as though he had built an אשרה, a grove for idol worship (Sanhedrin 7). The Choshen is an instrument to obtain forgiveness for the sin of perverted justice, whereas the ephod is meant to obtain forgiveness for the sin of idol worship. We know this from Hoseah 3,4 who describes the absence of properly constituted authority in Israel in the same breath as he bemoans the absence of ephod and teraphim. Such absence was due to the sin of idol worship committed by Israel. The חושן, breastplate, was permanently attached to the ephod in order to discipline the sinners who passed by, since the Torah legislates that people have to be reminded by hearing about public execution for this crime in order that they should become afraid (Deut. 13,12 et al). We have a principle that if an object must be destroyed for legal reasons, then even if it still exists in an undamaged form it is legally considered as having been destroyed and must not be used for the pupose it had once been designed (Chullin 89). Being afraid of the penalty of idol worship has the desired effect. The exposure to the sight of the High Priest wearing the ephod on his back and the חושן in front over his heart, there would be a constant reminder of this warning to all who saw the High Priest.