I've read a lot of books in Rathleton and tried to do research on numerous fansites and I believe that at the moment were unaware of the use for key 0001. Right now we all can't figure out how to get this key as it seems it only appears in rathletons safe which is closed off to the public as far as we're aware. I like to think the key is a play off of bonelord language as 1 means Tibia according to A Wrinkled Bonelord. I know back in the day key 0000 used to be rumored to open up all the doors in Tibia as a mapper was greedy, when Cipsoft found out they eventually replaced the key with no use. Maybe key 0001 because it has many forms opens up all rooms in Tibiia or at least the safes that are locked.
Sources:
Tibiaqa: is there a key that opens all doors?
Tibia Fandom: key 0001