This is what I found...
the "vomeronasal organ", also known as the "Jacobson's organ".
The Jacobson's organ is located in the roof of a dogs mouth, and is an actual olfactory organ.
The vomeronasal organ is mainly used to detect pheromones, chemical messengers that carry information between individuals of the same species, hence is sometimes referred to as the "sixth sense."
Simply put, dogs lick things, including your feet, to collect samples of what they smell to be processed by the vomeronasal organ.
Just because neutering a dog causes them to not care about what they smell (pee spots), does not change the WHY they licked them.
Dog Talk 101: Dog's Sense of Smell