Kinda strange people still arguing the real / fake issue after it has been cleared up earlier on the thread??!!??
;<)
What a "gag reel" does, is break the suspension of disbelief. It takes you out of the illusion created by the film and pulls you back into reality. It creates the unpleasant sensation for believers that their "faith" is not "real" because they have been abreuptly jerked out of the continuity of the drama.
J.R.R. Tolkien wrote an excellent essay about the suspension of disbelief and how to maintain it in writing fantasy. I'm sorry Peter Jackson did not include a gag reel in the extended cut of LOTR.