The Chest key is a members-only quest item. This version of chest key is used in The Dig Site quest. It is obtained from Doug Deeping and opens chest near the panning area on the River Salve, where players find ammonium nitrate.
There is no use for this key after the quest, and is safe to drop.
However, if players wish to use the ammonium nitrate to create chemical compounds to damage themselves, they should do the drop trick to obtain multiple copies. Banking the keys does not allow you to get another, it must be dropped.