If a Florida residents works in Georgia and receives a W-2 for Georgia taxes, why would his accountant need to know how many days he worked in Georgia? Wouldn't he just pay the taxes based on the W-2?

As a FL resident GA can only tax you on wages you earned while physically present in GA. So if you spent some of your work days out of GA (perhaps working from home, or at a client's site in FL), then GA can not tax you on your wages for those days, and so you would report a lower amount of GA-source wages on your GA income tax than is shown on your W2. That's why the accountant asked about it. Of course if you commute to work in GA every work day then you would indeed have to pay tax on the full amount as reported on your W2.