Why is mayonnaise used in some cake mixes what do it make it do to the cake?

Answer it make it soft to eat

Mayonnaise is made of egg yolks and oil. There's usually some salt and lemon juice in it as well but the main ingredients are egg yolks and oil. Using mayo in a cake is a quick way to add eggs and oil and since it's already emulsified together, it helps keep the cake moist.

It gives the cake a nice moist texture.

Yes, the Mayo adds a nice texture to it. Some use it as a substitute when they have no eggs! =)