A steel key ring is one of the rewards from the One Small Favour quest. It is useful because a number of keys can be placed onto it without taking extra inventory spaces. It is not required to take keys off of the key ring to unlock doors with them.
The key ring can hold most keys (excluding ones that disappear after single use, such as shade keys, muddy keys, HAM storeroom keys, and crystal keys). Keys that can be added to the key ring have a right-click "add to the tool belt" option. To date, the key ring can hold 11 keys in the following order:
It used to be highly recommended that players use the drop trick to get multiple key rings because if lost in the Wilderness, all attached keys would be lost as well. However, the key ring now is on the player's tool belt, and thus is safe in all situations.
The key ring previously held 13 keys, but after an update removed the Weapon store key from the game, it can no longer be stored on the key ring. After the update which moved the key ring to the tool belt, the enchanted key cannot be added to the key ring and thus it now holds 11 keys.