Decomposition of the body in the earth (after burial) is the slow oxidation of the body tissues.
Cremation, on the other hand, provides rapid oxidation.
No casket is legally required for cremation, just a simple container, which is strong enough to hold the body. This could be a box of rough boards, pressboard, or heavy cardboard.
Some crematories accept metal caskets; most require the container to be combustible.
If the body is cremated ...
1. The remains can be stored by the family