The reason for sifting, even just before adding liquid, is to prevent any clumps from forming. Flour that hasn't been used for a while and has gotten damp, depending on the weather, how you store it, and where you live, can greatly effect the portion of the flour that has been exposed to the air.
Someone who lives in a rainy/coastal city such as Seattle is going to have a different texture to their stored flour than someone who lives in a dry climate, such as Phoenix. Sifting is necessary in order for everyone to achieve the same result.
"Food is our common ground, a universal experience." - James Beard