...Let [their censers] be made into hammered plates as a covering for the altar, for they offered them before YHWH, and they became holy. Thus they shall be a sign to the people of Israel.
Numbers 16:38

Bronze represents judgment. Because they followed wicked leaders, the people were separated from the atoning power of the altar by judgment. Traditionally, the altar was a hollow framework filled with dirt before this, and the sacrifices were offered on the surface of the dirt. Afterward, the altar is filled with dirt, but covered in bronze. This is a picture of our restoration to YHWH only being accomplished by way of a mediator who is judged in our place.