When you rest, or leave your batter on the side/in the fridge the starch molecules in the flour are absorbing the liquid in your batter. This causes the molecules to swell and gives your batter a thicker, more viscous consistency. Any gluten formed during the mixing of your batter is also getting time to relax, and air bubbles are slowly working their way out.
This resting step ensures a thin and uniform structure to the finished baked good. Because the gluten had time to relax, the texture is delicate rather than chewy.