The cocktail parties put pressure on the couple to conform to societal pressures, even while the marriage is falling apart. They are a direct representation of what they should be and what they should not be, how they fail and how they succeed. The guests are not fooled by the pretenses that Edward tries to pretend that Lavinia has been called away. Even Lavinia knows that the guests wouldn't be fooled. Interstingly though, they don't care. They are alienated from each other, trying to make a human connection, believing lies or ignoring their falsity to not have to get involved. The cocktail parties highlight that with the going ons of the guests and the fact that they pretend to care, pretend to do right, when in reality they don't. Look at the end of the play, when Edward and Lavinia hear about Celia's death, and still go on preparing for the cocktail party.