The only reason a DM rolls dice is for the sound they make.
Rule Zero: "The GM is always right."
—Urban Dictionary...but sometimes, he's our cheating bastard. While Video Games are run by a cold, inhuman computer, Tabletop Games can be more flexible because they spring from the mind of living, breathing Game Master (who may or may not be cold and inhuman.) The Random Number God's power can be broken, plot-devices can appear to save or bedevil the players, encounters can suddenly become harder or easier and, depending on whether or not the game is Serious Business, in-jokes can abound. There are even specialty "Dungeon Master's Dice," which are more-or-less the same... but blank on every face! This can be a blessing if the players want to send their adventure screaming Off the Rails or eschew the usual Fighter, Mage, Thief party structure for something more flavorful. A nice cheating bastard can make sure things stay fun for the newbies and maybe give the characters a little help when they've had a run of bad rolls. Unlike that bastard, the computer, a good GM will mainly fudge the rules to preserve the vastly more important Rule of Fun and Rule of Cool. However, this trope has a dark side. If the nice GM takes things too far, the players' victories can seem preordained and hollow. And Pelor help you if a Killer Game Master is a cheating bastard... you might run up against a band of omniscient orcs that fight like they were trained as Spetsnaz. A sudden Ass Pull might save his Villain Sue, or you could miss a twenty-foot-long dragon because you Failed a Spot Check. If a particular player gets annoying, his/her character might get a Pit Fiend dropped on them. (Hell, the entire party may fall victim to falling rocks if the GM's feeling especially vindictive.) And remember those blank dice? A computer can't feel spite (as far as we know), but you may yet come to miss the Random Number God. But take heart! It can be nice to Earn Your Happy Ending, and it certainly feels good to have your characters triumph in this GM's mad world. It can often be more fun to be killed by this GM than to survive the entire adventure of a GM that's too nice.