If I call for backup, or someone to pick up a phone call, etc. and no one responds, the Closing TL starts calling people out by name to do whatever I requested, she doesn't wait for me to ask her for help. Backup for the checklanes isn't optional. If the SETL has to ask another TL to call people out, those people already failed the store by not responding to the first call.
A DBO does have a lot of work to do, but it's not as urgent as ringing up guests and getting them out the door. Service at the front end has a much more direct impact on sales. Guests that have to wait in long lines are prone to say, "Fuck it", and leave entire carts full of merchandise behind, which your DBO can then reshop rather than ring up, while also losing out on the sales for said items.