hand counts work in areas with low population, but in cities it would be impossible to complete on a timely basis. This will allow Trump to show an initial victory in rural counties because they can finish counting sooner.
additionally, requiring both machine counting and hand counting is also deliberate attempt to inject chaos as they are almost certain to not exactly match, and be used to promote distrust in the voting system.
If there is also a time requirement, then voters in Atlanta area are essentially disenfranchised, since there is no way to get enough hand counts completed for such a population.