Any scenario where the key is not ephemeral and you're handling client's input (can expect exploitation). If you have a good enough separation/sandboxing, any exploit wouldn't be able to steal the encryption key, or other private data, even if the exploit worked.

For handling encrypted secrets, this is popular as a HSM idea. You authenticate to a black box which does the crypto for you, but the key can't be extracted. Sometimes HSMs even have a physical tampering / self destruction protection.