The McKees appear only in Chapter Two of The Great Gatsby. Why does Fitzgerald bring them into the novel?

You could support your answer to this question with several speculations. First, the McKees add to the atmosphere of the raucous party at Myrtle's apartment in contrast to the tense, more subdued party at the Buchanans' in Chapter One. Their presence at Tom and Myrtle's party helps show the sharp difference between Myrtle and Daisy.  Daisy would never associate with rough, "classless" people like the McKees, but Myrtle fits in easily with them. This is ironic because Daisy did not marry Gatsby who could not "fit in" with Old Money Society, but she does marry someone who associates with people who have far less class than Gatsby.

Secondly, the McKees provide some subtle information to Nick, the novel's narrator. Nick observes the interaction between Mrs. McKee and Myrtle and recognizes the fact that Tom has been having an affair with Myrtle Wilson long enough for them to have an apartment together and for her to know her apartment neighbors.

Additionally, the presence of the McKees in just Chapter Two of the novel represents the shiftless society of Fitzgerald's era. Most people in his society did not form lasting relationships; they simply flitted from one party to another, seeking to fill their insatiable appetite forentertainment and something nameless.

Finally, later in the novel, Tom is critical of the guests at Gatsby's party--he does not believe that they are significant enough to warrant his presence at the party, but he thinks nothing of hosting a party to which he invites a freeloading, ordinary couple like the McKees. Fitzgerald takes the opportunity here to highlight not just Tom's hypocrisy but also the hypocrisy of the Old Money folks.