why did fitzgerald add the detail about the dog biscuit to the description of the party at myrtles apartment
The purchase of the dog reinforces the impulsive nature of Myrtle when she is away from George's influence and Tom's readiness to indulge his lover's every whim and desire. Myrtle immediately dotes on the dog, arranging the overcrowded apartment to make room "for a box full of straw and some milk." The elevator operator who becomes the errand boy in obtaining these supplies adds the dog biscuits when he returns, possibly recognizing that the dog would be hungry as well as thirsty.
Myrtle demonstrates her appreciation of the thoughtful gesture later on, complaining about the poor service delivered by the elevator boy and by the hotel service staff in general. "I told that boy about the ice." Myrtle raised her eyebrows in despair at the shiftlessness of the lower orders. "These people! You have to keep after them all the time." Fitzgerald is making a point about the inconsiderate nature of ungrateful people, particularly those pretending to be something they aren't.