The Nominal Ledger, sometimes known as the General Ledger, is necessary if you wish to produce Nominal reports such as the Trial Balance, Trading and Profit & Loss and Balance Sheet reports.
Whether the Nominal is On or Off all transactions are posted to the Nominal in the background. If the nominal has not been setup, i.e. all categories assigned to a nominal account, bank accounts assigned to a nominal account etc., the data posted to the nominal will not be useful.