Why do they use both egg yolks and whole eggs instead of just using whole eggs when making chocolate lava cake?

If I have left over egg yolks, I make french toast with them. It's f*ing divine.

Egg yolks make a cake/cookie much more tender and rich.

Their lovely fats also tenderize the proteins in the dough.

Yolks make the cake more rich.

Eggs help provide structure to the cake. An egg yolk will also act as an emulsifying agent, helping the batter come together more smoothly