The main purpose of a flour sifter is to aerate the flour, separating the granules and making it much easier to mix into a batter. Unsifted flour may have lumps that may be difficult to incorporate, causing the baker to over-mix the batter and creating too much gluten.
In addition to preventing over-mixing, sifting flour can aid in the prevention of gluten in other ways. Sifting flour before adding it to liquid ingredients can allow fat to coat the flour more efficiently, reducing the amount of water that comes in contact with each particle. Since water and flour work together to create gluten, this can moderate the amount of gluten formed and the toughness of the final product.