In his book, Searching for Memory, Daniel Schacter puts forward a curious description of memory.
According to Schacter, memories are encoded in the brain as engrams - essentially a neuron firing pattern - that captures certain elements from the experience. Certain kinds of encoding are more likely to promote higher recall - specifically, elaborative encoding that allow you to integrate new information with what you already know.
This explains why succesful ads use referent systems - as Adliterate has pointed out - by making our brain process information and linking it to things already in our heads, there is a much better chance that we will remember it.
So brand experiences will build brandgrams in our heads. So far so good, this all feels pretty logical. But then he veers off. What he suggests is that the act of remembering isn't really a recollection - it is a new experience.
"The cue combines with the engram to yield..an experience that differs from either of its constituents."
So the cue, the piece of communication, combines with the brandgram to create a new experience that "differs from either of its constituents". People are not simply experiencing the communication but the gestalt of the communication and their pre-existing brandgram.
Now that's what I call consumer created content.