Grease is told through three alternating POVs: Danny, Sandy, and Rizzo. Danny is always singing about Sandy. Sandy is always pining from Danny. If it were just the two of them, we'd never see anything other than close-ups of John Travolta and Olivia Newton-John. That wouldn't necessarily be a bad thing…but it might get boring after a while.
Danny and Sandy are so preoccupied by one another, that the rest of the movie is filtered through Rizzo's POV. For example, we know what Rizzo thinks of Sandy, but we never get Sandy's opinion of Rizzo directly from Sandy. Sandy is too busy singing about Danny to even let Rizzo register.
In the section on Setting, we talk about how Rydell High tries to be wholesome on the surface, like Leave It To Beaver. Danny and Sandy are the white bread, but Rizzo is the meat of the sandwich—her POV is what gives the movie its flavor.