Have you ever taken a bite of cake and tasted baking soda in one of the spoonfuls?
When you just dump in the flour, baking soda, baking powder, salt, cinnamon, etc, these ingredients aren’t evenly distributed in the flour and there is a high probability that one bite will have more baking soda than another. Adding such minimal amounts of an ingredient into the batter makes it really difficult for well-balanced distribution among the whole batter.
Mixing it with the flour makes sure that all the batter will get an equal amount of the leavening agent, spice or salt.
This is such a simple step that also has another function – fluffing up the flour and getting rid of all the lumps.