Fix Compacted Flour. Flour will compact over time (and during shipment). You could sift the flour to fluff it back up. Or, you could just stir it before measuring and be sure to spoon the flour into your measuring cup in order to get a correct volume measurement.
Remove Unwanted Material. Yes, sifting would also remove larger pieces or bits of chaff. It would also remove insects. However, neither of these are problems with modern store-bought flour. If you grind your own flour (my sister-in-law does this), then you may still want to sift it, though.
Mix Ingredients Together. Sifting can also be used to mix other dry ingredients into flour. I bake quite a bit, though, and I've never had a problem with just using a spoon or whisk to mix dry ingredients together.
In summary: don't bother. Just use good measuring techniques and stir your ingredients together well. No one likes a lump of baking powder lurking in a muffin!