Mixing baking ingredients separately Why would one need to mix the dry ingredients (flour, salt, soda) separately and before mixing into the liquid (butter, sugar) than just mixing everything into one bowl at the same time when making cakes? What would be the result if done ?
By mixing the dry ingredients separately and whisking them a bit before adding them to the liquid, you are making sure that the baking soda/powder/salt, gets evenly distributed throughout the flour.  Also, the flour will be able to absorb the liquid easier and more uniformly without the flour becoming over-worked.

From my personal experience, when I didn't do it this way my cakes seemed to be drier than cakes I had made when I combined them as instructed.  But this is from my personal experience, and others may have had different experiences.