Protected Periods protect existing, dated transactions, not allowing them to be changed.
Protected Periods still allow the addition of new transactions with a date in the period.
Your bookkeeper or system administrator might choose to Protect last month so there are no accidental changes to exiting tansactions, but that new entries can still be added.
This might be necessary if there are Vendor Bills that have not been entered and should be dated correctly for accrual and aging purposes. Please check before making changes.