Sifting (or just whisking) breaks up clumps that form when flour sits for a while. It also adds air and volume, so a cup of sifted flour and a cup of unsifted flour are two different quantities. Read recipe instructions carefully to determine whether sifting is needed: For “a cup of sifted flour,” sift first, then measure; for “a cup of flour, sifted,” measure first, then sift. The exception: cake flour, which should always be sifted.
Watch how to sift flour without a sieve: