The element within the joke that makes it funny, one that someone normally figures out for his/herself, is made obvious by being verbally described by the individual.
Saying this phrase leads to mixed results. Some may be annoyed by it and feel that it ruins the joke. Others may find it humorous because the element is somewhat funny in itself.
Fred: It's funny because... the mousetrap kills the first one.
Friends: <sighing, shaking heads>
Dave: A guy goes to his doctor for a checkup and the doctor tells him that he needs to stop masturbating.
"Why's that, doctor?" the guy asks."
"Because I've gotta finish this exam."
Craig: It's funny because... he's masturbating in front of the doctor.