Daisy serves as the point of contention in the conflict between Gatsby and Tom. She is the prominent symbolic figure in Gatsby's vision for his future. Again on a symbolic level, Daisy represents wealth.
Most simply, Daisy should be seen as the character driving the action of the novel. Though Nick is the narrator and Gatsby is the principal character, Daisy is the character that puts things into motion. Daisy invites Nick to dinner and initiates him into the "high society" world he finds in the east.
Daisy also serves as Gatsby's motivation, symbolically and practically. (She is the reason he throws his lavish parties. Gatsby hopes one day she will show up at a party and they will "naturally" meet again.)
Daisy is also the character that kills Myrtle, bringing the novel to its climax.
Clearly, Daisy's functional role is quite significant in the narrative.
On a more conceptual level, we can see Daisy as a representative of wealth. She is the aim of Gatby's pursuit and she is an example of the wealth that Tom takes for granted. She is directly aligned with this idea at one point when Gatsby describes her voice.
“Her voice is full of money.”
As the object of desire in a narrative where money is associated with a society's deepest desires, Daisy comes to represent the fulfillment of a materialistic American Dream.
Her whole careless world revolves around this illusion: that money makes everything beautiful, even if it is not.
Seen in this regard, Daisy is central to the development of the novel's themes relating to material ambition, social striving, and distorted value systems.