Celebrities are the sum of their parts: all of the ways that they appear in public, every film or television show in which the star appears, everything that is said about them, every picture that is taken and made available for public consumption. Together, these bits of information, pictoral evidence, and gossip form the celebrity's image, i.e. what he/she "means" or signifies within the realm of popular culture.
Award shows provide a primary means of celebrity image-making. Through dress choices, red carpet interviews, reactions to jokes at their expense, and acceptance speeches, the Golden Globes, Emmys, Grammys, and Oscars all add texture and prestige to a celebrity's image.
Except when they don't. A well-chosen gown, hairstyle, or significant other can land a celebrity on "Best Dressed" lists, but the poorly chosen ones are the ones we remember, from Bjork's infamous swan dress (image inflection: she's weird) to Gwyneth Paltrow's ill-fitting pink ballerina dress (image inflection: she's fragile and loves Ben Affleck. At least at the moment). An acceptance speech at the Oscar's can even turn into the defining moment of a star's image -- Sally Field's "They Like Me, They Really Like Me," for example, or Halle Berry's tearful acceptance for Monster's Ball.
This past year, the Oscars helped add texture to a star image that many people thought they knew well, namely, that of Natalie Portman. The first real "damage" occured when she won the Golden Globe for Best Actress and gave a weepy, sacchrine speech about her "Love," recent fiancé, and choreographer for Black Swan, Benajamin Millepied. The sentiments were repeated at The Oscars, where her pregnant belly and thankfulness for "the role of her life" (that of a mother) challenged her existing image, which had been characterized by her profanity-laced appearance in a viral SNL video, quirky boyfriends, attendance at Harvard, and her history as a precocious child actor.
The two acceptance speeches fanned the flames of existing gossip concerning Portman and Millepied's relationship: he had svengalied her; she was under his spell, he was a total creep, etc. Which is all to say that an appearance at the Oscars has tremendous power -- an award may increase a star's value within the industry, but the "performance" at the awards ceremony itself can forever alter what the star "means" to his or her audience.