Faced with a question, a purely common-law court will look up what the court said last time it was confronted by a similar question. Meanwhile, a purely civil law court will look up what the most relevant statute says about the question and interpret it as it thinks best.. Since each system has weaknesses, most legal systems today use a mix of both civil and common law, with legislators creating the overall framework of statutes, while judges fill in the gaps using common law methods based on precedent. Under this system, citizens can look at statutes to learn the baseline of the law, and then refer to past cases to understand how judges have previously ruled.
The analogy to a gamemaster in a tabletop game should, I hope, be clear. The game designer is the legislator; the game rules are the civil law; the citizens are the players; and the decisions of the gamemaster about grey areas in the rules are the common law. A gamemaster running a game like Savage Worlds or Basic Fantasy will end up acting mostly like a common law judge, forced to make rulings about particular situations without written statutes. In this case, precedent matters a lot. Fairness demands precedent.
Fairness Demands Precedent
To prove this point, let's illustrate what happens when precedent is ignored. Imagine that you are running Basic Fantasy, a rules light game modeled after the classic 1980s editions of Basic Dungeons & Dragons. During a desperate retreat, Marcus, your party's fighter, wants to jump across a 15' chasm to safety. "Fighters jumping across chasms" is not covered by the rules. You decide that this is a test of heroic agility best resolved with an ability check against Dexterity. If Marcus rolls less than his Dexterity he will succeed; if not he will fail and plummet to his death. Marcus rolls a 9, less than his Dexterity of 12, and succeeds.
Next round, Quintus, the magic-user, decides he too wants to escape across the chasm. You again consult your rulebook and note that "magic-users jumping across chasms" is not covered by the rules. You decide that this is clearly a test of herculean strength, and demand an ability check against Strength. Quintus, with a Strength of 7, fails the roll, and his player demands to know why Marcus got to roll against Dexterity but he had to roll against Strength for the same task.
What can you say to this criticism? That you're the GM, your word is law, and it's your right to rule however you like on situations not covered by the rules? That there is no written rule stating which attribute is to be used in resolving the success of jumps, so this is completely fair?
Let's now imagine that a couple weeks have passed, and the party must now, once again, jump across this chasm. You once again check the rules and again see no game mechanic specifying the chances of success. You announce that each character has a 2 in 6 chance of falling in, but otherwise they jump across successfully. Morne, who has both 18 Dexterity and Strength, demands to know why he now has a 33% chance of falling in, and not a 10% chance. You shrug in. "There's no rule that says it has to be an ability check," you say.