The marinade won't slide off the chicken: Mayonnaise is creamy and thick because it's emulsified.
So when you add it to a marinade, it helps the marinade to adhere to the surface of the chicken, rather than run right off.
Also, since the mayo thoroughly coats the chicken, it prevents the marinade from burning right off when it hits the heat.
Chicken will be golden brown and juicy: The egg proteins in mayo brown the chicken and lock up moisture.
The best part, the chicken won't taste like mayonnaise. This is important for me!
When combined with the mayonnaise, the pesto flavor and color both remained brighter and more flavorful once cooked.
Mayonnaise may never be the star of my show, but it is a very important supporting character.