You do not have to sign a reaffirmation agreement or any other kind of document to do this. You do not have to pay a secured claim if the debt is discharged, but the creditor can still take the property. Even if a debt can be discharged, you may have special reasons why you want to promise to pay it. For example, you may want to work out a plan to keep your car. To promise to pay that debt, you must sign and file a reaffirmation agreement with the Court.
Reaffirmation agreements are under special rules and are voluntary.
They are not required by bankruptcy law, or any other law. If you are an individual and you are not represented by an attorney, the Court must hold a hearing to decide whether to approve the reaffirmation agreement. The agreement will not be legally binding until the Court approves it. If you reaffirm a debt and then fail to pay it, you owe the debt the same as though there was no bankruptcy. The debt will not be discharged and the creditor can take action to recover any property on which it has a lien or a mortgage.