"And he was placed in a coffin in Egypt” (Bereishit 50:26). In earlier times, people were buried in coffins of wood, stone, metal, or clay. However, in order to fulfill the verse that says “to dust you shall return” (Bereishit 3:19), they would modify the coffin to create a direct connection to the earth. Either the bottom of the coffin was removed, or, minimally, holes were bored in the bottom or side.
These holes served an additional purpose. They prevented defilement (tumah) from ascending heavenward. For the law is that if there is a space of a tefach (8-10 centimeters) between the body and the top of the coffin, and there is a hole in the side of the coffin, the coffin is not considered fully sealed, and thus the defilement cannot spread.
In Israel today, the dead are buried in the earth without a coffin. This is a more ideal fulfillment of the verse “to dust you shall return.” Indeed, many sages in the past objected strenuously when important people were buried in coffins. Nevertheless, in the Diaspora, people are generally buried in coffins. It is also the practice to bury fallen soldiers of the Israel Defense Forces in coffins, for obvious reasons.
It is forbidden to derive any benefit from coffins and burial garments (tachrichin). Therefore, one who finds boards in a cemetery must not move them, as we are concerned that they might be from the coffin of a body that was exhumed.