After completion of the associated quest, if you have unlocked the entire Hub track, this item can be retrieved from the Quest Item Storage chest at the Quest Point Caravan if lost or destroyed. Otherwise, a new one can be obtained from Grip.
Since Jagex has made it possible to complete the quest without a partner, keeping this key is no longer necessary. Before then, it was a good idea to keep it so you could assist someone needing help - this was the only quest you could help with after completing it yourself. There is no longer a need to keep this item after the quest has been completed. As such, it can be destroyed or kept as you desire.