[danShumway]

I can think of setups that would allow this, but I don't understand what the advantage would be over sharing keys.

Honest question, if I have a key that can unencrypt all of your data, why is it important that it not literally be your key?

[extropy]

It's useful to have a master key that decrypts everything. For each message that means encrypting for two keys - the recepient and the master key.

The obvious drawback being the huge damage when a master key is inevitably leaked.

[scarejunba]

Because then I need a way of giving you my key that doesn’t give the bad guy the key.