> If the ledger is tracking transactions that occur somewhere else and for some reason a transaction is entered incorrectly then immutability will prevent the ledger from being corrected to reflect reality.

You just enter a new operation correcting the issue. If you can't do in in the external ledger, you do it on your own side and then cancel your correction using a new operation when the external ledger is finally corrected.

The point is auditability. You want what's inside the system to reflect what's actually happening.