Like the introductory portion of any good work of literature, including theater, "Cotton Blossom" carries the weight in establishing the setting, mood, themes, character development, and conflict. It is all these literary elements together that tell the story of Show Boat.
Without setting and mood, the story offers no orientation to support the themes and conflict(s) of the story told. The themes expose the importance and significance of the story told. They also expose universal truths applicable to contemporaneous and subsequent audiences who listen to the story told. Character development provides the motivations of the actants of the story told. In other words, it explains why the characters do what they do and why they do or do not learn what is needed. The conflict arises out of character development and is the crux, or the heart, of the story told.
A couple of specific examples will illustrate these points. Steve is sulking. Julia tells him not to be silly as she could never be interested in Pete.
JULIA: Don't frown, Steve. Smile for your public.
STEVE: If I ever catch that feller Pete talkin' to you again, I'll break 'im in two.
JULIA: Aww. Don't do anything foolish, Steve. What could a man like Pete ever mean to me?
Pete becomes angry when he sees a gift he gave Julia on another woman's blouse. Captain Andy tells Steve to "Smile, darn ya. Smile." The conflicts between Julia and Steve, Pete and Julia, Pete and Steve, Captain Andy and Steve are thus all established during "Cotton Blossom." These setup an import racial theme related to Julia having "black blood" but "passing" as white.
Another important racial theme, that of freedom and privilege, is set up during "Cotton Blossom" with two eloquent shots of the townspeople happily welcoming the Show Boat. The first shot shows an excited crowd of men and women, but they are all white. The second shows an excited crowd of men and women, but they are all black. This racial and moral segregation theme is introduced in "Cotton Blossom" and elaborated upon later in "Old Man River."