Also by sifting flour ensures no foreign oblectsobjects are unin the flour; bugsflour...bugs, toys, coins, large flour lumps, et alchocolate chips, cupcake sprinkles, pet fur, etc.
If you have small children that help you bake, or pet friendly kitchens, you'll be surprised what you can find in your baking ingredients!!
Bugs sound gross but they do happen, especially in old house kitchens, or if you use a tub or other container to hold your flour, better to find out if you have a problem before you start mixing!