Whisking the eggs and sugar on high for a few minutes will add a lot of air to the brownies, which creates a lighter texture. That’s why the recipe calls for folding in the ingredients - you don’t want to lose the airy texture you’ve just created. While these brownies aren’t cakey, the air helps the brownies keep their lifted shape, even with all their density. This step also contributes to the shiny, crackly crust that makes a brownie, a brownie!