What is egg yolk?

The yolk is fat plus an emulsifier (lecithin) with very little water, so it behaves nothing like the white. Adding an extra yolk instead of a whole egg enriches the dough and increases chew without adding the water and protein that egg whites contribute for structure and lift. Whites push a bake toward cakier and drier; yolks push it toward richer and chewier. That trade-off is why some cookie recipes call for "1 egg plus 1 yolk" — you get the binding of a whole egg plus the extra fat of a yolk for a softer, chewier crumb.