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.
Getting it uniformly mixed is kind of the real reason. You want to get it all mixed as much as possible before you get the flour wet and it starts to develop the gluten. Regardless, you have to mix enough to make it uniform.
For cakes (mentioned in the question), its largely about avoiding 'over-working' the batter. You want a small, even crumb so (unlike breads) you're trying to avoid gluten. Mixing, like kneading, promote gluten. You want to make it so that as soon as the flour gets wet, you're doing the least possible mixing to incorporate the ingredients. You also want to use the batter quickly. Luckily, modern cake flour is very low protein. If you're using standard AP flour instead of cake, you really need to be very careful about overmixing.