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A Lawful Good Rules Lawyer can also be a valuable thing to have in your gaming group if one or more of the players in your gaming group cheats or doesn't RTFM.
Even when the rest of the group is on the level, the fact that a Rules Lawyer will, by definition, know all of the rules can make them useful for a gaming group as the ultimate natural language database.
They will attempt to employ every loophole, every odd circumstance, and every footnote they can.
However, this trope does come in a positive variant referred to as the Lawful Good Rules Lawyer: they always stick to the rules, no matter how bad it might be for them personally.
The teenaged Marshall was also something of a mischievous troublemaker.
His greatest high school accomplishment, memorizing the entire United States Constitution, was actually a teacher's punishment for misbehaving in class.
Gaming groups must, by necessity, be a mutually policing force.
Sometimes a Rules Lawyer is necessary when the GM is repeatedly sending waves and waves of homebrewed mooks who are immune to everything except the powers of that one super duper archmage the GM has been writing a novel about for the last seven years and if you try to fight them without the archmage GMPC, you die. In this instance, the Rules Lawyer is one check against GM misbehavior, as in a healthy gaming group, the GM is answerable to the players as much as the players are answerable to the GM, because it is everybody's game.