Why is the trip to Memphis to buy Whisky important to this story line? The author could've brought it to climax w/o the trip.
This story demonstrates the destruction of nature as result of the encroaching progress of "civilization." Faulkner includes this scene to demonstrate the juxtaposition between the urban and the rural. While in Memphis, Boon and Ike stand out among the city folk because of their dirty hunting clothes. Boon, especially, looks like a wild man, and in the space of fourteen hours he gets drunk twice.
However, they are still men, and although they represent nature while in the city, they are determined to conquer nature, as shown by the hunting scene of the next day.