With this approach, the butter coats the flour particles, creating a barrier that slows down gluten development. Gluten won’t start to form until the flour comes into contact with the water from the egg whites, so coating the flour particles with butter before the eggs are added inhibits gluten development, making for less rise and fewer air bubbles.
Just as important, since the butter isn’t beaten with sugar, less air is incorporated, which also translates to less rise and a flatter cake rather than a domed one—a boon for streusel-topped cakes and decorated layer cakes.