Hi Gigi! There are two reasons to call for sifted flour in a recipe. The first is to get rid of any clumps in the flour so it mixes evenly with the other ingredients. But more importantly, a recipe will say to sift the flour to make sure you get an accurate measurement. This is because all flour (even “presifted” flour) will become compacted in its package, so 1 cup of unsifted flour will actually be a little bit  more (or maybe a lot more, depending on how compacted it is) than 1 cup of sifted flour. Sifting the flour and then measuring (if that is what the recipe says) means that your measurement will be the closest to what the recipe developer intends. (And it’s important to note that “1 cup sifted flour” is not the same as “1 cup flour, sifted.”  The first means that you should sift the flour, then measure out 1 cup. The second means that you should measure out 1 cup of flour and then sift it.)