If my debt was charged off, then I don't owe it and don't need a bankruptcy, right?

Wrong. If you have debt that shows as charged off on your credit report, this means that your debt was removed from your creditor's balance sheet, as a loss. You are still responsible in terms of having to pay it back. The charged off debts are often sold or sent to collection agencies. On the other hand, a bankruptcy discharge means that you are not liable for the debt anymore, at all. That is the power of bankruptcy.