Skipping the resting period for pancake batter can result in tougher pancakes with less rise and a less even texture. Resting allows the gluten to relax and the leavening agents to fully activate.
When you mix pancake batter, you're developing gluten. Gluten is a protein found in wheat flour that gives structure to baked goods. Overmixing leads to excessive gluten development, resulting in tough, chewy pancakes. Resting the batter allows the gluten strands to relax, leading to a more tender pancake.
Furthermore, most pancake recipes include a leavening agent like baking powder or baking soda. These agents create carbon dioxide bubbles that give pancakes their light and fluffy texture. Resting the batter allows these leavening agents to fully activate and distribute evenly throughout the batter. Without resting, the leavening may not be as effective, resulting in flatter, denser pancakes.
Finally, resting allows the flour to fully absorb the liquid ingredients. This hydration process contributes to a smoother batter and a more even cooking process. If the flour isn't fully hydrated, you might end up with pockets of dry flour in your pancakes.
Don't overmix your pancake batter, even after resting! A few lumps are perfectly fine and will disappear during cooking. Overmixing after resting will re-develop the gluten and undo the benefits of resting.