The flour is sifted to break up lumps that may form during storage and to obtain a uniform, loose structure. By passing through the sieve, the flour separates and becomes lighter, so it mixes more evenly with the other ingredients. This helps to reduce the formation of lumps in the dough and achieve a more uniform texture.
Sifting is especially useful when preparing biscuit doughs - butter cakes, sponge cakes, cakes and other plain cakes where an airy structure is important. It is also useful when using cocoa powder, powdered sugar or other ingredients that tend to form lumps.