Together with -E, this shows a balance for both used and declared
accounts (excluding empty parent accounts, which are usually not
wanted in list-mode reports).
This is somewhat consistent with --declared in the accounts and payees
commands, except for the leaf account restriction.
The idea of this is to be able to see a useful "complete" balance
report, even when you don't have transactions in all of your declared
accounts yet. I mainly want this for hledger-ui, but there's no harm
in exposing it in the balance CLI as well.