In The Great Gatsby, Nick's claim of objectivity followed up with judgmental language sets the tone from page one that he'll be an unreliable narrator. This idea of an unreliable narrator creates a sense of ambiguity within the novel.
At the very beginning of The Great Gatsby, Nick says his father told him, "Whenever you feel like criticizing someone ... just remember all the people in the world haven't had the advantages you've had." Then Nick goes on to say that because of this advice he's "inclined to reserve all judgments." Then he goes on about how Gatsby had this "extraordinary gift for hope" he will not see ever again. But then, soon after this proclamation about Gatsby, Nick goes on a diatribe about the awfulness of Tom saying that he reached his pinnacle playing college football and now "everything savors of anticlimax."
So the question is, why is it important to understand that Nick is an unreliable narrator? Nick's reliability and his obvious bias toward Gatsby should cast doubt on how each event is presented. Nick's refusal to judge Gatsby's affair with Daisy comes off as intellectually dishonest while his refusal to judge Tom for Myrtle stinks of cowardness, not objectivity. Yet he kind of just skims over his relationship with Jordan, which was clearly more serious than he presents. He clumps her in with Daisy and Tom in order to excuse his outright dismissal of Jordan. He also excuses his inaction in the second chapter of the book—his refusal to criticize Tom for striking Myrtle and his possible homosexual experience with Mr. McKee ("...I was standing beside his bed and he was sitting up between the sheets, clad in his underwear...")—by beginning this chapter with the excuse, "I have been drunk just twice in my life, and the second time was that afternoon."
From page one, Fitzgerald presents Nick as this flawed character telling this flawed story. At the very least, Nick's friendship and admiration of Gatsby and his obvious distaste for Daisy and Tom should call into doubt the actual telling of the story of that summer.