Just cause I don't like Mayo...the family does, for the record Tom, well most of the Family, just not Smokin' Jr -- the oil gets to him
The Mayo thing and the Mustard thing. Two of those legends in the Q'in world.
The Mayo thing was popularized as a trick used on the competition circuit to keep chicken/poultry moist. You coat the chicken with Mayo and since Mayo is almost ALL oil (you knew that didn't you) the oil helps keep it moist throughout the cook.
The Mustard thing (which you didn't ask about, but has been in the forum a few days lately) is where you coat a brikset or butt with Mustard and then sprinkle with Rub. The thought it that people believe it helps the rub stick better. There's a lot of vinegar in the mustard and after cooking you don't really notice a "mustardy" taste.
Hope that answers the question.
Smokin'