It's much like wearing a safety harness when up in a tree stand, even without a law, it's plain common sense.

With bow season, early muzzleloader, special youth and senior days, squirrel, and fall turkey seasons running concurrently with small bird season, there is great likelihood that at some point during the season, you will be in range of someone else's weapon. It's difficult to imagine why anyone wouldn't want to stand out given that fact.