In a raw egg white, proteins are like a ball of yarn—and when you whip an egg white, the proteins unravel similar to how a ball of yarn unwinds. Egg whites contain protein and water, and the water component is what allows these proteins to unwind.
How so? When you whip egg whites, it is much like blowing soap bubbles: Pockets of air are surrounded by a thin film of water and, in this case, unraveled protein. You get large, unstable air bubbles that will deflate easily after a period of time.
Gradually adding sugar to egg whites creates a more stable meringue. The sugar is hygroscopic—meaning that it attracts water. Therefore, it makes for a stronger and longer-lasting protein structure by slowing the drainage of water away from the liquid surrounding the air bubbles. The result is smaller but more stable air bubbles. With the addition of sugar, the whipped egg white foam is now stable enough to be baked in the oven to form a meringue. Sometimes the resulting meringue was too brittle and dense. It was hard to cut after it cooled. So we needed to create a stable meringue but one that was also more delicate.
The solution was to add the sugar in two parts—half at the beginning and half at the end of the recipe. Adding less sugar at the outset results in larger, more delicate air bubbles. When baked, the resulting meringue is stable yet easier to cut without breaking. The sugar added at the end is for sweetness.