Plywood on roofing now?

[Durian Fiend]

Does anybody know why the county is requiring plywood sheathing on roofs now? Is it a structural thing? I read the code and can’t find it anywhere. I figured I’d ask here before calling the county directly. Thanks in advance.

Plywood is not strictly required.  
We are building a small house with 2x6 TNG planks for the roof deck.

What's the advantage of these planks over plywood? I guess it will be your finished ceiling inside, so this is being done for aesthetic purposes.