Baking powder is double acting — it reacts first in contact with liquid and again in the heat of the oven. Letting the batter rest helps fully hydrate the baking powder and helps your muffins rise higher in the oven. This is a tip I picked up from Hummingbird High's sky high blueberry muffin recipe, and while these muffins won't rise nearly that high, the resting period will give them an extra boost and a nicer texture.