With a VA, you are not saddled to a 9-5 workday. They work around your schedule and are there when you need them. Rather than assigning eight-hour shifts, you can develop a schedule that maximizes output while keeping costs in check.
And, in the unlikely event that you’re dissatisfied with your VA, you can simply end the contract and hire a different assistant.
It’s not always that simple with a full-time employee!