Usecase:

1. Created a backup key while provisioning a smartcard (e.g. Yubikey / opengpg card)
2. Card got broken / lost / etc, so a new one has to be ordered and will have a lead time to be able to provision with the key from 1 (if possible)
3. converting the backup key from 1 in the meantime would make it possible to keep using the gpg functions / access encrypted mail etc, only with the loss of physical devices