Typically, the mole helps the players win the final task (Kathryn in Three Rooms, Bill rejecting the dossier in Top Secret, Frederique in It Takes a Thief, and Angie with that picture game). I think Mark initially planned on helping them win the final mission, BUT I think Mark needed to keep the $75,000 out of the pot because he tried to sabotage the clock-bomb game, but Craig ended up cutting the purple wire instead of blue+red, so Mark had to go for the dossier.